FaceBook is no longer limited to campus students of US universities - there are over 27 million active users on FaceBook now and a good portion of that based may be located outside the US. [I know so many Orkut fans here who are slowly shifting loyalties to FaceBook.]
FaceBook has several paid services like FaceBook Polls, FaceBook Gifts and the more recent FaceBook flyers which require members to pay in order to use them - the only problem is that FaceBook has limited payment options.
FaceBook Flyers looks like a very effective medium for advertising - you can show up an image ad with a good 200 word description on various Facebook profile at the fee of $10 per 5000 impressions which is like $2 CPM - pretty decent rate considering the size of the banner and that it also allows images.
Unfortunately, if you are outside the US, there's no way for you to pay for advertising via the FaceBook Flyers service - it won't accept international credit cards - you have to be residing in US with a Visa, Mastercard and American Express credit or debit card to use this service.
By limiting the payment options, FaceBook may be limiting the number of advertisers. The day they add Paypal or even Google Checkout, the growth in small advertisers will be massive.