It was replaced a few years ago by Meebo - and then Meebo apparently stopped hosting chats on websites outside their own.
That said, you might have chosen a good time to come back! The staff have been asking for users' suggestions on how to improve the site, and some of the older users have been suggesting that they bring back certain features that are long gone. So if you would like them to bring back Fanpop Chat, now is the time to suggest it!
Here's a forum you might want to suggest it in (and any other features you want back, or perhaps ideas for new features):
link
Also, check out these two spots:
link
link
Sorry it's not really an explanation as to where the chat went or why, but if you want it (or anything else) back on the website, now's the time to make your statement!
And welcome back to Fanpop!