According to an article on mashable (Send & Receive Money From Twitter & Facebook Friends With Dwolla) if you live in the United States, you can now send money directly to friends on facebook and to people who follow you on Twitter in a very simple and low-cost way.
Dwolla (www.dwolla.com) links directly to your bank account and allows money transfers to other users through twitter, facebook or its own interface. It is now available in all of the US. The cool feature is that transfer costs are 25 cents per transfer, regardless of how much or how little money you send.
Dwolla is a peer to peer payment platform which allows users to exchange money effortlessly through Twitter or Facebook.
Use Dwolla to send money to anyone directly from your bank account. You can also shop online without dealing with a money order or costly credit card fees.
Hub pages allow you to accept payments on your own landing page! Share the URL with anyone and you are ready to accept payments.
The drawback: that there are no international transfers, only US for now.
A previous report on Dwolla on this site: Dwolla low cost transfer system to take on bank, credit card fees