With an estimated 500 million users worldwide, Facebook has become one of the most popular web services in history. To die-hard fans, the thought of losing their Facebook account would be unthinkable. Facebook has quickly become an integral part of their everyday life. And all of it for free.

But everyone knows there is no such thing as a free lunch. Sooner or later Facebook will have to start charging to use its services just like any other company. Right?

The fear that at some point in the future Facebook will charge a monthly fee has millions of Facebook fans worried, and some are in a panic.

Just doing a quick search on Facebook for "Facebook charge", or "Facebook free" turns up hundreds of groups denouncing any attempt by Facebook to start charging for access. Some of these groups have managed to garner thousands of concerned users. Even Facebook CEO Mark Zuckerberg has said that rumors of whether or not Facebook will start charging for its services is actually a bigger concern for many fans than privacy issues.

So are these claims grounded in reality? Are any of them really justified? Well, according to Facebook spokesman Larry Yu "We have absolutely no plans to charge for the basic service of using Facebook." And when you stop to think about it, this makes perfect sense. After all the company's mission is to make the world a more open and connected place. Charging for access runs counter to this goal.

While all of this makes for great PR, there is a more materialistic motive behind a free Facebook – profit. But how can they make a profit if they're giving everything away for free? There's no free lunch in this world, right?

The genius behind Facebook is that it generates billions a year in revenue by selling the attention of a big audience to hungry advertisers. Millions of eyeballs translates into (hundreds of) millions of ad dollars. Imposing a monthly charge would quickly shrink membership and in so doing cut into revenues.

In 2010 Facebook is estimated to have revenues in excess of $2 billion, more than double last year. With numbers like that it is doubtful that they would do anything to jeopardize their user base. They want you online as much as possible. The easiest way to do that is to make the service free.

While Facebook may continue to charge for optional bonus features, such as games and gifts, it is in their interests to keep the basic service free. Even though you might feel like you're getting a great service gratis, in reality the more you use it the more money Facebook makes. Your free lunch is effectively being paid for by the advertisers.

So dig in and stop worrying. Facebook will probably always be free.