| | Re: on Facebook?
No - because there it's seen as a "place". A chat room is seen as exactly that - a room that you can go into and come out of when you want. It's metaphorical of course, but what you do parallels entering and leaving a room exactly. So you go "into" and "out of" a chat room. But you see something "on" a site.
Interestingly, forums like this one are sort of half way between the two - so you sometimes see people talking about posting "on a forum" and sometimes "in a forum".