Facebook, Twitter, or any social network isn't something that can sell itself. It needs you, and preferably every single one of your friends, ever.
I recently joined a new social network, Path, and I cannot find a single person I know. I like the idea of Path. I like the limit of 50 friends and its simplicity, but if I cannot find a soul on it, then what's the point of using Path? I want to share moments with people, not with myself. I do that all day long, and trust me, I get sick of my crazy self.
The goal of any social network is to connect people. If a social network can't do that, then what's the point?
So, what do you market to get people to join your network? Sure, features are one thing, but really, what else can network offer beyond sharing text, pictures and videos? People.
That's why you see your friends and possible acquaintances constantly popping up in the sidebar on Facebook. That's why Twitter tells you "Who to Follow." It's why LinkedIn shows people you may know. Because these people and what they offer to the network are the product. Everything else is secondary to providing access and information on the people.
Social networks are only fun when they are social. That's why Path looks like a barren wasteland of ones and zeroes to me. It needs humanity. Without people, social networks are just networks.
And who wants to be a part of that?