Pretty sneaky, Facebook (Nasdaq: FB).

The social networking giant is ramming its @facebook.com email addresses down your throat. Forbes -- and then Lifehacker -- noticed yesterday that the email addresses featured on user profile pages had defaulted to the @facebook.com names that it began issuing two years ago when it let folks create their own vanity URLs.

Nobody seems to be using Facebook's awkward email platform, but now the company has a way of forcing the issue with as many as its more than 900 million registered users.

Facebook users can opt out of this, but they never opted in.

That could be a problem for Facebook, but for now it's probably more of an opportunity.

Free ad-supported email is a major part of the portal playbook. Google (Nasdaq: GOOG) has Gmail. Yahoo! (Nasdaq: YHOO) has Yahoo! Mail. Microsoft (Nasdaq: MSFT) has Hotmail.

These dot-com giants have been cautiously watching Facebook's heady growth in recent years, and rightfully so. The more time that folks spend on Facebook, the less likely that they are to be found elsewhere. Why hit up Google for a roofer when a status update on Facebook can generate trusted leads for someone to repair your leaky roof? Yahoo!'s popular news and entertainment hub becomes less necessary when Facebook -- and Twitter -- are breaking all of the major headlines.

Microsoft, on the other hand, has a minority stake in Facebook and a healthy marketing relationship, but it also can't be happy with the sneaky ploy by the social networking juggernaut to get its email platform off the ground.

Monetizing email hasn't been easy for any company, but with Facebook promoting the vanity usernames as emails, Facebookers will now have one more inbox to sort through.

It will be interesting to see how users will react to the company's brazen move. Will they appreciate the reminder that they have a Facebook-tethered email that they can use to filter out spam? Or will they resent that their default emails have been changed without their consent?

My money's on the latter, leaving Facebook with a fair deal of explaining to do.

A world of opportunity
Burned by the Facebook IPO? Find out about the recent tech IPO that you should be buying. It's a free read, so what are you waiting for? Check it out now.