When it was first introduced, I was wondering why in the world Facebook would incorporate a chat feature. Over the past 2 years, Facebook has gone from light and fun to heavy and repetitive. By adding yet another 'application'/functionality, they were just bogging things down even further.
I didn't care to use it at first, but now I'm using it quite a bit. The main reason is that when I see a friend online, it find it better to just send a chat message and get a live response - most of the time.