And here comes the big stick.

Facebook are cracking down on alternative micro-payment systems, and from July 1, 2011 only their own currency, Facebook Credits, will be allowed for those games on the social network that want to appear on the Facebook Games dashboard, and also take part in the gaming-focused advertising.

They’ll be happy to set up an exchange for an in-game currency, such as Farmville FarmBucks to Facebook Credits and back again (reports TechCrunch), and games using the login system Facebook Connect aren’t being forced down this route either, but you can be sure that Facebook are now ready to get everybody into their system.

We’ve already mentioned the high transaction tax of 30% being paid to Facebook on currency transactions from “real” world currencies, but less has been said about both the lock-in factor (a Facebook game is now intrinsically linked to the site), and the ambitions that Facebook have with all those credit card details on file.

It should be no surprise that the big players in the Social Gaming scene are already signed up to use the credits in multi-year deals. I don’t think it’s going to be that long before digital goods outside of games will be available to buy through credits. Let’s not forget that in the early days one of Facebook’s constant revenue streams was the “gift” that you could place on a friend’s wall. Purely ephemeral, a little gif on the server, but oh so profitable to the tune of $15 million a year in 2008.