I don't know, but they do seem to need a lot of developers, and developers don't come cheap.

Summingbird and Heron come to mind as quite-probably-very-expensive projects that seem to solve some very Twitter-specific problems. My completely uninformed, biased, pulled-out-of-thin-air suspicion is that said problems are largely self inflicted. It reminds me of Fog Creek's longtime insistence on using their own in house programming language.