Three things lacking in current social media tools.

Recognition that content and identity shift based on the audience it’s targeted at. I’m different variations of myself with family, with friends, and with close friends. Usually the separation available is an ‘allowed to see’ or ‘not allowed to see’ binary.
A compelling mobile experience, AND a compelling desktop experience. Twitter is clearly (and intentionally) a better experience for me on a mobile device. Facebook is a desktop tool, with a reasonably functional mobile version. These experiences don’t need to be identical, but I’m looking for a solution that makes a platform compelling in both arenas.
Function as a collaborative tool. Nearly all social media tools I’ve seen have been under the umbrella of conversational publishing, or recording social behavior for distribution and posterity. Social structures help us build things together, too. I haven’t seen meaningful innovation on that front since wikipedia.