Thank you for all the information here.
So far I've only used the 'forums' approach...this is a good suggestion and thread!

Facebook flash games.

It depends on what you want to do with Facebook, if you intend to use information from Facebook (eg user's friends, newsfeeds, invite...etc various features) then you'll have to use Facebook's APIs. Which aren't very good atm. It's possible, but you'll have a lot of trouble trying to set it up. Certain stuff will require more effort...

If it's just putting your flash game up, then it shouldn't be much of a problem. There is also a 'developer' application that allows you to do that.

One thing to probably note is that Facebook games are generally designed to be played for short bursts of time (which should be kept in mind when making your game) People mostly play Facebook as a short 5-10minute distraction from work. Also, doing the 'spamming invites to all your friends thing' is REALLY SHITTING ANNOYING. =X

But yeah, there are a lot of FB games that have made money and it's worth researching on those. Flufffriends in particular uses paypal directly. However, it is not really a 'flash' game.

I guess that is one of the problems with flash games...in most cases, people expect them to be free.