And people have been making your point ever since Facebook opened up to the general population. But MySpace, even in its heyday, wasn't nearly as big as Facebook was a year or two ago.
It's not the number of people using it that matters so much as who those people are and how they use it.
Myspace was most popular among high school kids, indie music culture, and people looking for dates. Many people in their 20s and 30s started using it to keep in touch with old friends and make new contacts but it never grew beyond that. By the time they implemented the news feed, it was too late. Facebook had already gotten the momentum.
Now, people (and organizations) of all sorts have Facebook profiles and pages. Families, adults, colleagues, friends, new acquaintances, all use Facebook to stay connected. Sure, the kids will almost certainly find something new to play with soon, but that won't matter anymore.
Facebook is huge, and now pretty clearly shows enormous potential to continue growing.
