[rahimnathwani]

I'm not sure I understand the example you gave:

1. What purpose would escrow serve in this example? The user trusts the social media site, and the amount is small (like $1). So there's no need for a third party to hold/control the funds and determine when they are released to one party or another.

2. A spammer can just wait until day X+1 to start spamming.

[roywiggins]

Metafilter has a $5 signup fee, which works in approximately the same way. If you get kicked off for spam (or any reason, really) you're out the $5.