Summary: if your product isn't useful (or doesn't have utility in the economic sense) it's harder to charge for it.

Duh.

The unfortunate conclusion of this is that social media may just not be that useful to people. DHH even made this point at his startup school talk - saying that if people aren't that willing to pay for your app, maybe it's just not that useful. I believe they've also stated elsewhere that when they moved Basecamp (or was it backpack) to a B2B product instead of B2C it got way easier to make money.