I'm very aware if I'm on Facebook, knowing that my friends are also aware of it. (I know that you could determine who was online before the institution of chat, it just wasn't as obvious or accessible.)
I'm very aware of who else is on Facebook, if they might want to talk to me, if they have been on there a while, if they're logging on from work.
I'm also cognizant when I click out of Facebook - was someone just about to initiate a chat session? Now that may be because my Facebook interactions are usually quick hits- 15 seconds to see if there are any new messages or notifications or friend requests, maybe a minute or two if I'm going to play Scrabulous.
I've preached before about the dangers of universalizing one's own experience, so I won't make the assumption that my neuroses are universal. But on the other hand, I'll assume I'm not the only one who's become more self-conscious about using Facebook and I'm wondering if this makes the site less useful because anything that makes you self-conscious inevitably becomes less fun. And I'm guessing the answer depends on how much FB chat is adopted. If everyone ignores it, the self-consciousness will go away. If it becomes more common, then self-consciousness increases.
Until, of course, someone invents an app that lets you use Facebook chat without actually being logged on to Facebook