Three months after opening its APIs to the world and inviting developers to build applications for the surging social network, Facebook has decided to close those doors just a wee bit.

The reason? App developers are using spam tactics and viral marketing to the point of influenza --- making many Facebookers sorry they installed the things in the first place.

Take the "Likeness" application, developed by somebody named Peter Louis. This silly little applet lets you take a quiz to find out how much you are like/unlike your friends and/or celebrities. (You and Britney both love getting trashed on Cosmos? You must be soul mates.) Stupid and harmless, right?

But like most FB apps, you can't install Likeness without sharing your basic profile information. And once you do, it nags you to invite all your friends to join the party and install the developer's other apps.

Here's what one Facebooker has to say about Likeness:

"This thing SUCKS. Everytime I deselect my all my friends because I do NOT want to spam everyone every 5 minutes, it won't tell me my results. It simply errors out. Of course, every time I include just one friend to spam it doesn't error out. Convenient. I am not doing any more of these."

Here's another:

"Hey "Peter," quit making your apps function by being dependent on each other, you're turning into a major douche here.....There is no excuse for this behavior, and it calls into question the motives behind the creation of these applications.....Is this just a huge Data-Mining exercise?"

And a third:

"Please ... don't send me emails about a friend wanting to see how alike we are. Apps should only email users if they've added the app, otherwise you're just viral spam. I'm blocking your app because of this."

That's the trouble with random third-party developers. You invite them in for tea, and a few weeks later you discover that the fine silver is missing and your daughter's knocked up.

Facebook has also changed how it measures an app's popularity. Instead of merely ticking off how many people downloaded the thing, FB now counts the number of active users it has. So, though millions have downloaded Likeness, only some 444K suckers still play with it.

Exactly how the new, slightly less open Facebook will play out remains to be seen. Neil Day, CEO of MediaMaster (which has a Facebook music app that doesn't abuse the rules), says it's all good:

I think this is the right move for Facebook. It helps users find valuable apps, and limits obnoxious behavior on the part of their app developers. I think it will benefit people who are developing valuable apps, but it will make it much more difficult to rack up huge user counts through spamming and trickery.

In an entry on Facebook's official blog, senior platform manager Dave Morin writes:

"We hope to shift the balance more in favor of good apps, which we think in the long term is good for everyone. Users will get better applications, and users will be able to put more trust into applications, thus spurring further adoption.... and we will continue to block applications which behave badly and ... to iterate on our automated spam detection tools."

All of these are welcome changes. The question is, why didn't they think of them before? Are the Facebook folk really that naive?