IM+ works for facebook chat as well, it also does aim, yahoo, msn, google talk, jabber, icq and myspace chat. The price is kind of stiff at 50$, but it's on sale for 20 in the app store until the 10th.
I have several of the chat IM progs installed and IM+ is the most useful- it has most iM apps but importantly skype. Nimbuuzz is free but buggy and does not link with the message inbox. Beejive is fast and pretty and trillian ditto - beejive drains battery and trillian has a memory leak.
Individually - I like whatsapp and gtalk but they all work ok
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