What kind of ASP.Net application will be probably popular?

I plan to do a web service application by ASP.Net, but I don't know What kind of ASP.Net application will be probably popular.
I konw that the social bookmark (such as http://www.furl.net/index.jsp) is very popular, and that web service application is very good.
I don't think my web service application for social bookmark can exceed http://www.furl.net/.
So I hope to do some other ASP.Net web service application, any ideas for me? Thanks!