After all, the headline sites who make the most from this business are worth hundreds of millions - and some are even looking to launch on the stock market. Even if they don't entirely base their business on Facebook (in many cases, they are available on - or partner with - other social networks too) the spread of users suggests that they're heavily invested in it.

• Back in November, Electronic Arts bought social gaming site Playfish, in a deal we are now told was worth around $275m.

• Meanwhile Zynga, another developer of popular games (like ) has already taken more than $200m of venture capital.

• Other companies making applications include Slide (also closely linked with MySpace but funded to the tune of $78m); Mindjolt (recently bought by MySpace co-founder Chris DeWolfe, funding not public); and of course FriendFeed (bought by Facebook for around $50m).

• On top of that, there's a huge number of companies like the aforementioned Smartdate, Plancast ($800,000); and a whole bunch of companies pushed forward by Facebook's own $10m fbFund.

That's just the start.

What other companies do you know that are reliant on Facebook? How big do you think the Facebook economy really is? Stick any information you know in the comments - once we've got an idea how big this is, we can start thinking about whether it's a viable ecosystem, a bubble or a house of cards.

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