Pidgin is arguably the de facto open source instant messaging client. The biggest advantage you have using it is the multi-protocol support which allows you to connect to AIM, Yahoo, MSN, Google Talk, Jabber, IRC and a few others I’ve never even heard of by default. Plugins add additional services and features. Here is a look at the services I currently connect to all at once:
Pidgin is available for Linux, Windows, and Mac here.
The Twitter plugin can be found here.
The Facebook one is available at this site.