I think it's that sites initially provide valuable services for free, at a loss. Users love them, and so they grow. But loving something and being willing to pay for it are very different.
The site expects, eventually, to stop losing money and even maybe earn a little profit. So after they've become popular and everybody seems to love them, they start trying to charge a little money here and there, or otherwise find some way to monetize.
Then the site realizes that all those users that love them so much, don't really love them enough to pay or to tolerate other irritating forms of monetization.
So then the site has a choice:
A. Continue losing money forever
B. Keep up the monetization efforts, despite knowingly irritating and losing your customers, partners, content producers, etc.
And most reasonably choose option B, even though they know it's the death knell.
The site expects, eventually, to stop losing money and even maybe earn a little profit. So after they've become popular and everybody seems to love them, they start trying to charge a little money here and there, or otherwise find some way to monetize.
Then the site realizes that all those users that love them so much, don't really love them enough to pay or to tolerate other irritating forms of monetization.
So then the site has a choice: A. Continue losing money forever B. Keep up the monetization efforts, despite knowingly irritating and losing your customers, partners, content producers, etc.
And most reasonably choose option B, even though they know it's the death knell.