MySpace. Atrociously designed pages stuffed with pictures, videos and music that typically starts on its own.
Millions of people created their own pages on MySpace. What was the appeal? Not quite sure, but they reached critical mass, real people you know were using it and it was growing on its own. Then one fine day, the party was over and MySpace crumbled into semi-nothingness (thanks God, what an eye sore). It still exists, but who would create a MySpace page now?
What caused MySpace to crumble? I have read many theories, but the bottom line is that they managed to piss off their users (I gather their advertising deal with Google and the cheap tactics had a lot to do with that). They grew arrogant and decided that they could do whatever they pleased. And they were wrong.
Is Facebook going the way of MySpace? Facebook is the opposite of MySpace in many ways. You can't f*** a Facebook page, and a Facebook page always looks just like another Facebook page. There is no design issue to speak off, it just works. Videos will always show the same way, pictures are in a proper lightbox...It's hardly what anyone would call pretty, but it works. The apps infrastructure is great and obviously the gaming platform it developed really took off.
But they are starting to piss off their customers nevertheless and they don't seem to give a broken html tag about what the users think. Users want more privacy? The official answer is F*** Off. The users want better control on who sees what? Same answer. Ads? ...
FML stories (stories of people complaining about the mundane tragedies of their daily lives), Facebook is in the spotlight nearly every day: "Someone said I was a bitch and all of my friends 'liked' it, including my mom" or "My boss wrote on Facebook that I should not bother to go back to work."
A major failure of Facebook, IMO, is that there is no way to really break down your online personae in different, separated groups. In real life, you have different types of contacts which you interact with in very different ways and each has a very different access level to your personal life. You don't deal with your parents the same way as you deal with your friends or with your boss or with your work colleagues or... and even within these groups, you discriminate: You don't talk with your dad the same way you talk with your mom.
But on Facebook, all the people you know pretty much have the same access by default, and as Facebook gains widespread adoption, the friction between the real life model and the Facebook model increases. We are starting to get stories about people getting fired or sued over Facebook posts. Relationship break ups on Facebook are pretty common (now your boss, your coworkers and your mom know that your girl slept with your "best friend"). Without going to these extremes, the end result is that people become more and more careful with what goes on Facebook, resulting in boring banalities and generic, sanitized, interactions, and eventually, people are going to look for other places to interact with each other and Facebook will be dead.
Unless there is a major change in the way Facebook works, my uneducated guess is that it will take a dive within the next 3 years.