It seems so weird that we all happily pay $5 or $10 a month for a bunch of random services, but a central platform like Facebook or Twitter can't even charge a buck?
My own personal gimmick ideas for twitter, that will never be tried, would be "premium" which gives you abuse reporting that actually works (and maybe some other stuff), and a system to enable people to pay for premium (or part thereof) for other users.
There is also the fact that it can be hard to see the overall gain when each interaction is very marginal. For example, updating your timeline with a photo and a status by itself doesn't seem worth paying for even if the cummulative effect over an entire year could be very beneficial for your networks.