It's still trivially easy and often very enticing to bail out of Facebook entirely. I still don't see anything that Facebook is doing that successfully locks in customers, not even in the way that Amazon locks in book-buyers and certainly not in the way that Google becomes most people's default approach to Web search.
Moreover, if every user loses money for Facebook, what's the financial model of the company? Even if I like Facebook, if I'm not providing it with cash flow, when it does it have to bail out of its business?
It is not as easy as it seems. I use facebook because it easily allows me to see what is going on in a lot of people's lives. There are no huge technical or design hurdles to beating Facebook. The problems are social.
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.
Don't compare Facebook to Twitter. It's enormous in comparison. Twitter may be getting lots of press, but my friends who've tried it have nearly all abandoned their accounts.
Moreover, if every user loses money for Facebook, what's the financial model of the company? Even if I like Facebook, if I'm not providing it with cash flow, when it does it have to bail out of its business?