It makes sense. Their biggest running cost right now will be Amazon. Up to a certain size S3 makes sense as it would cost more to run the infrastructure yourself and it might be less flexible (scale wise) as a smaller operation can't afford to have an awful lot of spare capacity lying around. As the userbase continues to grow though, there will come a time when running their own infrastructure will work out cheaper and more flexible and they might be able to profit from it the same way Amazon do (remember: Amazon's "cloud" services came about from wanting a way to make profitable (or at least cost neutral) use of all their spare capacity in the quieter ten months of the year), so they would be daft not to at least have some embryonic plans simmering away on their R&D people's back burners.