Now that Facebook has reached 500 million active users, the obvious question is how much bigger can it get.

Given Facebook’s tremendous growth over the past 18 months, would it be unrealistic to suggest Facebook could soon have one billion or even two billion users? As important, if Facebook has one billion users, what does that mean to the Web’s growth and overall health?

What’s particularly impressive about Facebook’s growth is how it has continued the ride the “hockey stick”. Just when you think Facebook has peaked, there’s another growth surge.

Much of this growth has to do with Facebook’s expansion within demographics beyond the core 21-to-35-year-old audience.

Another key factor may be the growing corporate adoption, and how Facebook Pages have become an online staple for many companies, particularly business-to-consumer businesses.

This corporate embrace has seen them highlight and drive their presence on Facebook and, in the process, likely encouraged more people to join Facebook.

It is not unusual these days to see companies advertise advertising and marketing campaigns by driving people to Facebook rather than their Web site.

And as much as privacy advocates have their concerns about Facebook’s social graph, the more Web sites that become part of the Facebook ecosystem, the more people will climb on the Facebook bandwagon.

You put all these elements together, and it creates a powerful growth engine that shows no signs of sputtering. In some respects, Facebook’s growth could be a good thing for the Web by providing a powerful presence to counter Google’s growing dominance. Without a strong number two, Google could easily put a stranglehold on the Web with little resistance from consumers.

Of course, the Web’s health depends on a rich and wide of online players – big and small – so it will be interesting to see whether this ecosystem can survive and thrive in a world dominated by two Goliaths.