I don't think it'll be as successful as Gmail chat.
It feels strange to leave it on hours at a time, as you would with Gmail chat. It feels like you're somehow sending the message to your friends that you've got nothing better to do with your time then to sit around on Facebook all day.
I've always viewed Facebook as a guilty pleasure that everyone does. However, I don't feel comfortable advertising how much a participate in this guilty pleasure to everyone, or even increasing the amount of time spent on it.