Yeah this will work...
Since you have billions of people connected to the site why not use your brain zuckerberg and do an ebay / craigslist / paypal rip-off. Let people sell their shit on fb either locally or via an auction house and let people pay with face bucks.
Later on your facebook phone can be used to spend your hard earned face bucks at your local starbucks etc. etc.
Nahhhhhhhhh, lets just charge people to send messages.