What can I say about Facebook, that hasn't already been said?
I have had an account there for about 3 1/2 years. I joined it well over a year before Twitter and I still use both more or less equally, but for different purposes. Facebook is very definitely my family-and-friends network. So, my CV (which ought to be online by now, but isn't, so I can't point to it here) gives my account details for Twitter and LinkedIn, but not for Facebook. And I post work-related stuff there rarely, and only when I reckon that it might be of general interest to at least some people who have no part in the biomedical, higher education or science communication communities.
Aidan, whose interest in Facebook has dwindled as his enthusiasm for Twitter has grown, posted in July 2010 that Facebook offers "play versions of things that thrive better in a full-sized version elsewhere". I can see the point of that, very clearly, but I can also see the point of play versions.
Take photography. I have one Facebook friend who is an enthusiastic and very able wildlife photographer. The majority of his photos of beasts have their home on his Flickr photostream; Facebook is where he keeps photos of his own wedding. Another keen photographer who is currently using the 365 Project to record a photo a day for a year, nevertheless mounts casual snaps taken at parties on Facebook. My own photography is very much of the "casual snap" variety, and I want my family and friends to see it; Facebook will do.
And sometimes Facebook does rather better than that. Does anyone else with accounts on both run "Facebook-Twitter races", just for fun? The idea is to pose the same query to both sites and see which comes up with the answer first. Today, I had just such a query. I'm going to a conference in Barcelona in September and find that I have a spare morning - no more - between arrival and the start of proceedings. I've never been to Barcelona before; what will I be able to see in that limited time? I posted the question on Facebook and Twitter, and waited. Facebook won hands down, with half a dozen useful suggestions within a few hours; my tweet sank without trace. Why? Well, perhaps because my Twitter network is still rather small, and quite a few of my followers are institutions. Perhaps because they had serious things on their minds (#Libya has been dominating my Twitter feed all day). Or perhaps because (so far) relatively few of my followers are librarians. Any ideas?