Is NoSQL is suitable for Social Networking kind of applications

Good overall architecture and design is many times more important than simply picking a data technology; the latter won't magically make your site work. But yeah many of the leading NoSQL technologies would work fine, if your app design is good enough.

If you are mainly interested in navigating the network relationships, then I think it's fair to say that graph databases such as Neo4J and InifiteGraph are worth checking out. (Constant time navigation of links in the network.)