Social networking is the fad of the century, apparently. I partake in it to some degree. Actually I'd still find a static web page and some discussion on Usenet or some Other Place quite sufficient, but it looks like today anything without a GUI is just not done any more.
Although I resisted for a while, a friend and colleague finally made me create a profile at Xing. Alas, it was less helpful than expected when I tried to leave the job I had then. Still serves to present my extended professional information to anyone who is interested, but I rarely use Xing's other features. (Update: I learned that in between Xing shows only a very small part of the user's profile if you are not logged in as a member. Still better than LinkedIn, which shows nothing in that case.)
For a while I used Facebook, but more and more I found the changes to the -- already too complicated when I started -- privacy setting tools and concepts really annoying. I am convinced they are designed to confuse the users and make them finally give in to Zuckerberg's "post-privacy" ideas. I am still undecided if I should find these ideas just immature or outright dangerous. (I have been leaning more to dangerous lately.) Anyway, I stopped using Facebook. Twice.
Despite there being a certain segment of former colleagues on LinkedIn with whom I wanted to stay connected, I deleted my account there after the news of the acquisition of LinkedIn by Microsoft. (Update: I have created a token account there again, but only to be able to see others' profiles. I have nothing there myself besides a photo and the current occupation, and no contacts.)
When I write something of maybe a little more than ephemeral value, it goes to a blog at Dreamwidth. This is a rather unsorted collection of technical stuff (some bits about IPv6 at home when I had it new, for instance), books I have read, cooking stuff, and things.
Then came Google+, just like Facebook, but not Facebook. To do them justice, they have learned from the experience of other projects, including Facebook, and made a few things better. My presence there was mostly dormant during the last years before Google+ was closed.
With twitter I have a kind of on-again, off-again affair. Currently it has been "on again" for a while, and I try not to get overwhelmed again by all that stuff and people I am (potentially) interested in.