That's not quite what I meant. Sure you can chat directly fairly easy through Facebook, but communicate is more than just that. Keeping to the Facebook example, much of the 'communication' is done through user or group wall posts.
It is notoriously difficult to browse through, responses are sorted in non-intuitive ways, sometimes responses are hidden for no apparent reason, yet people regularly use it to communicate.
When people post status updates or pictures or whatever, they're trying to communicate, how many times have you gotten status or upload notifications from people you haven't talked to in years, but it never showed you your best friend's new baby pictures or something like that?
Twitter, It's designed around a character limit that strictly discourages longform communication. Yet, you get people trying to write blog posts using it, much to the chagrin of many HN commenters, and generally, the quality of most communication suffers greatly on twitter because of the inherent design.