Much of the appeal of Bitcoin (to the people who actually buy and sell things over the internet) involves relative anonymity.  They don't want to use credit cards that directly link them to a specific transaction.

The purchase of various substances.
Participating in various activities.

Online dating could reasonably fit that model for many people.  Some fraction of 40 million people could be a boost for Bitcoin liquidity and market penetration (if you will pardon the expression).