I think it depends on the site and vertical, but I don't think first always wins. Sometimes it does, but not always. Especially with disruptive technologies and the disruptive paradigms social technologies have introduced.
I could probably talk myself into either point of view regarding first to market or not.
On the first mover note. Craig's list is an example of a first mover advantage and win. eBay tried to compete with CL by building Kijiji and can't win. eBay couldn't catch them with a 26% equity stake it purchased through a disgruntled ex CL employee, proprietary knowledge it acquired through that equity, hordes of cash, and an army of developers.
I think the Kijiji failure can be summed up simply. They're competing on the same paradigm. Free listings. They thought they could catch Craig's List with a prettier, 'more intuitive UI. Craig's List is as ugly as a pipe wrench, but it doesn't matter. Kijiji is the same paradigm and a prettier UI doesn't solve a pain point.
In terms of first mover advantage, I talk to a lot of people who have business ideas and their test as to whether or not they'll execute on that idea is "has someone else thought of this idea"... You can almost guarantee someone has thought of it.
Even though it's cheesy (paradigm shift), I go back to the idea of the business paradigm. Is there enough of a shift to where that shift creates an enormous amount of value and alleviates a major pain point. If so, you're 10% there.. :)