That's slightly different. Other search engines were poor. They focused on the wrong things, and didn't actually give good search results in the way users wanted.

Facebook actually solves most peoples needs very well for a social network / casual gaming platform.

Any competitor will have to work for a few years just to get the features that people will assume it has. Then comes the massive job of moving people from facebook.

Consider ebay. Ebay actually really sucks. It's terrible. Yet everyone still uses it for auctions. Why aren't there any auction startups? Because it's impossible to get people to move.