Media is still talking about Twitter as if it wasn’t a business, focusing on the impressive tweet count, rather than its revenue. I wonder why.
Running a service like Twitter is way simpler than e.g. Facebook due to the nature of the transferred and stored content, so you can’t really compare them, despite the “social” tag.
While Twitter stores short messages that all fit in database rows together with their meta data in a relatively simple database structure, with no need for file hosting except for the small user icons, e.g. Facebook needs to transfer and store large files very effectively together with handling the now very complex database structure and overall functionality. I wouldn’t be surprised if Facebook is actually the most complex Web service in the world.
The only thing that really matters in the long run is of course how they will get profit or (if not, or even so) who will acquire them.