One obvious use case would be tipping. They could use BTC for this, but I don't think they will. For one thing, tx fees are pretty high, and will probably stay that way for a bit. For another, does Mark really want to pump the Winklevai's bags?

Imagine adding $1 in FB (FACEBUCKS!) to your "Happy Birthday" friend messages? $0.05 tips on cool pictures.

Then:
-Pay-per-click content?
-Affiliated merchants/advertisers? (10% off our new sneakers if you pay with Facebucks?)