Running an instant messaging service (such as AIM, Facebook chat, or Google Talk) isn't cheap. Companies running these programs need to make money somehow, so knowing what people are exchanging and selling that to advertisers / marketing types is one way.
What can they sell? How about every time someone mentions the latest action movie? Advertisers can know if they've played too few or too many of a given commercial. That might sound innocuous, but what about if you're arguing with your wife? Wouldn't you like to see ads for divorce attorneys? Customized search results sound like a great thing, but are only good for the companies that compile them. It might seem like paranoia, but the more information search companies can get about an individual, the more targeted their searches and advertisements. For Google and Facebook, that's most of their business.
Instant messaging service privacy reports don't paint a pretty picture and, as you might expect, Facebook is the worst.
Solutions? Many online services host their own chat servers (known as XMPP Servers), but another option for Windows is using Pidgin with OTR installed or Adium for Mac (with OTR installed by default). That way you can benefit from the fast, reliable commercial servers without giving up your privacy.