LinkedIn.com has recently added forum discussion to groups. The forum has added some dynamics and interactions into the professional social networking site. Other applications include Search on People/Job and Q/A. One of potential killer applications, I think,

would be providing a capability for everyone to build an interesting network that tries to aggregate certain aspect of ideas, technologies or whatever.

You can track people in the network closely. This would help people build more dynamic professional network. Instead of one-way job search, it creates a two-way matching mechanism and potentially tap into more passive candidates who may not be on the market but are interested in seeing opportunities.