Isn't the main reason that Facebook is winning that they are really, really aggressive about making people invite their friends? MySpace also grew by getting access to people's Inbox and spamming their contacts, but FB might have taken it to the next level.

I hardly ever log into, but FB seems extremely pushy to me. They constantly push people at me who might be my friends. Either I could invite them, or I could make them use FB more by befriending them.

Facebook is just a more subtle version of Farmville in that respect...