MySpace has several times as many accounts as Facebook and the gap is not closing quickly. If you go by the number everyone quotes, this is settled.
Go by how often people come back and it looks like a different market entirely.
The two sites are built on opposite assumptions about who you are online, and I do not think enough is made of this. MySpace is a page you decorate. You choose a background, you pick a song that plays at whoever arrives, you write a description of yourself, and the result is a performance aimed at people who do not know you. The customisation is the point, which is why the pages are slow and frequently unreadable, and also why a fifteen year old will spend four hours on one.
Facebook will not let you change the colours. Everyone’s page looks identical, which sounds like a limitation and is the entire strategy.
Because the pages are uniform, there is nothing to look at except what people you know are doing. And because it grew campus by campus, requiring an address from the institution, the people you know were already there before you arrived. You did not join a website and then go looking for friends. You joined a place where your existing life was already represented and largely accurate.
That is a much slower way to grow. It is also much harder to leave.
Leaving MySpace costs you a page you made. Leaving Facebook costs you contact with two hundred people whose phone numbers you never took, and the cost goes up every month you stay.
The registration numbers hide all of this, and I have complained about registration numbers before in another context. An account that was made once and abandoned counts exactly the same as an account somebody opens twice a day. The two sites are being compared on a measure that treats those as equivalent, and one of them has far more of the first kind.
Now that Facebook has dropped the requirement for an institutional email, the structure that produced the retention is gone and the interesting question is whether the effect survives it. My guess is that it does, because the graph is already dense enough to pull people in through their existing contacts rather than through a shared campus.
If that holds, the ranking everyone is publishing inverts within about two years, and the write-ups will describe it as sudden. It will have been visible in the return-visit figures the whole time, in a column nobody puts in the headline.