MySpace died because it became a minority player in a quickly growing landscape, at a time where alternatives were available.
To simplify, if you have captured over 50 percent of the entire english-speaking Internet population, sure you have a huge network effect behind you. But if that population count is multiplied by 100x and your numbers don't grow alongside that, your network effect is not as strong.
My wife was working there at their peak (and left at just the right time—I like to think that their decline was the inevitable result of her departure). They had such high engagement that they had a hard time selling enough ads to fill all the available ad slots.
To simplify, if you have captured over 50 percent of the entire english-speaking Internet population, sure you have a huge network effect behind you. But if that population count is multiplied by 100x and your numbers don't grow alongside that, your network effect is not as strong.