We've spoken a lot here about identifying and linking things and data about things. But 2008 will stay as the year of linking people. Clay Shirky exposed that brilliantly in his book "Here Comes Everybody"
As hundreds of millions of people, I've engaged in a few social networks this year. Not that I was not in some on-line communities before. In 1999-2000 I was Open Directory editor, since 2001 I've been a lunatic Wikipedian. But it was not those fancy Web *.0 social networks making billions of dollars. I've never been in Facebook or MySpace or the like, never going over this a priori reluctancy to join in noisy, aimless social chatter, exchanging images, music and recipes. No thanks.
Nevertheless ... I joined LinkedIn in January 2008, mostly to update my address book. At the time there were 17,000,000 people "In". Now the figures are over 30,000,000, or so they say. At the end of 2009 it's bound to be 1% of mankind. But LinkedIn is mostly an ego-booster. The main purpose of being "In" is very selfish : look at how efficient and professional, how many people I'm connected with etc. There are groups, but they have no real social activity as far as I can tell. So I keep my LinkedIn account for what it's worth : a free human resources directory.
In spring I had a try at Twine. With good marketing, they attracted a lot of smart, brilliant people. It was an interesting experience, but mostly unfocused and time-consuming. Twine has no particular purpose beyond conversation. I was quickly fed up with that, so I left.
My favourite these days is WiserEarth. I was very impressed by the clarity of purpose, the quality of information, and the spirit of the community. Of course the numbers are still small, less than 20,000 to-date. But you're most welcome to join.