Only sites with a lot of users, like Myspace, could quickly become a threat to Myspace. Facebook has surpassed a critical mass of users, so they probably would have time to respond to any technical challenges.And Myspace seems to have a different and less successful focus than Facebook that would need to be changed. I don't really care about telling everyone who my top friends are, designing my page, or using Myspace as a blog. Basically, I just want to quickly and easily find out the major events in my friend's lives. It does not matter how brilliantly designed a site is, I cannot adequately meet my needs with a site until it has a good portion of my friends as users.
So to go against Facebook successfully, a company would need to start in an extremely small niche and grow, or provide value unrelated to social networking. It is a lot easier to reach critical mass in a very small niche. And if you aren't solely in social networking, people are going to be using your network before it provides decent social value.