There is a prof* here who 'friends' his grad students on Facebook, then if he logs into Facebook and sees that they're on Facebook during the day he gives them a hard time about it.** Uh yeah. Excuse me, I just took a break from playing Bejeweled Blitz and cybersnooping on my evil ex and I noticed you're logged into Facebook, so get back to work, dammit!
As for me, I'm conscious of trying to set a good example, so I am not Facebooking at work, and all IM'ing is limited to short exchanges with my brother. Mr. X lurks in IM and Facebook occasionally but I either stay invisible or ignore him unless I'm actually at home and want to IM him.
*No, it's not my boss.
** What works for Mr. X also works for the boss. If you don't want to be seen in Facebook, stay out of chat.