Stowe Boyd writes in Darwin Magazine about The State of Social Tools. Among the things he talks about is the tendency of applications to tend toward converegence (if I use this app for this, then I also want to use it for this and this and this). Boyd says that there are basically four features of social tools:

Communication: instant messaging, e-mail, Web conferencing, streaming video and voice tools, and other messaging solutions

Coordination: calendaring, task and project management, contact management, and related technologies

Collaboration: file and application sharing, discussion, wikis, blogs and other shared-space technologies

Community: social networking, swarmth (digital reputation, also called karma or whuffie), group decision and other explicit community supports.