I wasn't cool enough to be in the first wave of invitations, but I have now got onto Google+, the Maybe Next Big Thing in social networks. It seems somewhat appropriate to mark this Momentous Event by writing a little bit about how I've used social networks so far, and some uninformed speculation about the impact of Google+
My initial reaction to social networks was suspicion. At heart I'm a misanthropic hermit with enough intelligence to put on a veneer of sociability. So I was never tempted by the early social networks like MySpace, and I still regularly refuse any requests to Link In.
I'd kept half an eye on Twitter, but was slow to get involved. Someone registered the name @martinfowler and started feeding my blog posts into it. I was worried that it may be a squatter, but it was actually a nice programmer who happily handed over the account when I asked for it. Eventually my colleague Doc List got me going on Twitter and now it's a regular part of my life. Shortly afterward I joined Facebook too.
I use the two networks quite differently, which is partly the function of my fate of being a geek celebrity. Twitter is a public facing stream, where anything I tweet goes out 35,000 followers. With Facebook I only link to people who I know personally. At a simple level I can think of Twitter as my professional side and Facebook as my social side - but that's a bit of a stretch since so many of my friends are also professional links.
Twitter is a fascinating social construct. It's a mix of features that I can't imagine someone would design up-front . The 140 character limit is one of those silly technology fiats, yet it works well. As Elliotte Harold tweeted of Google Buzz: "Favorite Buzz feature: I'm not restricted to 140 chars. Least fav Buzz feature: everyone else not restricted to 140 chars :-)". Similarly the asymmetric follow (35,000 follow me, but I only follow 300) coupled with mentions (which get my attention) works better than I would have ever thought if someone described it to me. All in all it allows me to keep a good sense of connection without drowning in the flood of text.
Almost all the people I follow on Twitter are people I know personally. I read pretty much all the tweets from people I follow or people who mention me. I check the stream pretty regularly, several times a day most days. I post a few times a day when at home, a bit less when traveling. I choose my public posts conscious of the fact that I have so many followers, so I try to post things that are interesting to my perception of that crowd. Mostly these are links to interesting articles, which naturally include anything I write. I reply a bit more casually as I see things from friends or mentions. I find Twitter a useful source of web links to read which supplement my regular blog feeds. I also enjoy the feedback on my own contributions that appear whenever I tweet a new post.
With Facebook, I know that people who see my posts are only people I know reasonably well. The other difference is that my non-geek friends don't use Twitter but many do use Facebook. I don't check Facebook so often, usually only once or twice a day. My posts there don't include geek stuff, mostly they are either interesting social things we are doing. One common thing I links to my photo albums.
One issue for me is how to deal with people who are on both services. My approach is to almost never post the same thing to both places, along the geek/social divide. But several of my friends copy all posts to both services. I'm still unsure how to deal with that, I could block them on Facebook, but then I'd miss some conversations.
I use a consistent avatar photo on any site that I use. People are used to recognizing faces, so I think it's only reasonable to use a regular face photo as an avatar. It's frustrating when someone posts and I can't remember who it is because I haven't learned to match their face to their name yet. 
Overall I've come to be very glad of both services. The nature of my life means I have friends all over the globe and it's good to keep some low-level regular contact with them. I was never one for sending off emails, and have always disliked phone calls. Posting short messages about what we're up to work well for me.
So how will Google+ fit into this? Of course I don't really know, there's no sensible way of predicting the outcome. The question for me is whether Google offers something that the current Twitter/Facebook combo does not. From the small amount of use I've put it too so far it looks well put together. Circles seem to me to be a very useful feature that matches how I see most people's social interactions.  (Although as George Dinwiddie pointed out - remember that Google (and Doubleclick) are in all your circles.)
On the whole I'm very happy with Twitter and will need a lot of persuading to shift off it. Facebook, I'm less excited about and could easily see me moving. The circles notion could be just right for that context.
But the key issue here is that it doesn't matter what I think or do, it's what all my contacts do that counts. I mostly have a Facebook account because people I want to keep in contact with have a Facebook account. If they don't move to Google+ I'm staying with Facebook even if Google+ is nicer. That's why Google+ has a huge mountain to climb, especially since I don't want to check Yet Another Social Network. I suspect there may only be room for one of Google+ and Facebook.
So far this makes Google+ only marginally interesting to me. It's rather like Wave was - a cool idea but of little use if nobody else is using it. What may be the difference is tying in with Gmail. I always got the sense that the Wave and Gmail teams weren't on speaking terms, hence their software never seemed to be either. Google+ allows you to link to people who aren't on Google+ but do have your email address, your posts go to these people as email. I could do with this already for some non-Facebook friends, and it could be an important booster to getting people to use Google+. In general there seems to be a determination with Google+ to tie in with other Google services. This may be the lever they need to shift Facebook, other than their obvious advantage, but it will still be a big challenge.
1: I'm sure that somewhere on the web is a really interesting article on the evolution of Twitter's features and their impact on social interaction. When you find it, please tweet it to me.
2: If you do use a face as your avatar, please use full face only (not head and shoulders) otherwise it will be too small to see. Also make sure there's enough light on your face to be able to see it properly at that small size.
3: I've heard some people say that circles are a big innovation. I find that astonishing, surely it's obvious that people interact differently with different groups of contacts? I think the fact that Facebook didn't do this is that they are fixation in their view of the world that people should be public - which also explains their regular missteps over privacy.