I think a big difference between MySpace and Facebook is that MySpace targeted indie bands and teenagers - people who aren't going to be thoroughly married to a platform.
Facebook started with college students that are a little less ephemeral, but has spread out to soccer-moms and grandparents. Those folks are far less fickle.
Even if Facebook loses the younger users, it will have a rock-solid userbase for decades. The company might be doomed, but this is going to be a very very long tail. Its downfall will look more like Sears than FourSquare.
Facebook was founded in February, 2004, almost exactly 10 years ago. A lot of those college students it started off with are already the soccer moms of today.
Other differences: Facebook had MySpace and other early social networks to see what they did wrong. In addition to just the general not-keeping-up-with-the-times, the excessive customization + bored teens = eyes bleeding & having to look at different places on a page to get the same info. Although it's gotten worse it's still cleaner than MySpace was during its heyday.
Also, Facebook has Zuckerburg. Whatever you say about him, he's smart and focused on keeping users, so there's constant tweaks to see what will keep them.
Facebook started with college students that are a little less ephemeral, but has spread out to soccer-moms and grandparents. Those folks are far less fickle.
Even if Facebook loses the younger users, it will have a rock-solid userbase for decades. The company might be doomed, but this is going to be a very very long tail. Its downfall will look more like Sears than FourSquare.