Five-hundred million's an awful big number.

It's the world's population divided by 14. It's the approximate number of people living in European Union countries. It's the combined population of Latin America, minus Columbia and Argentina.

The company now ranks fourth among global Web properties in terms of unique users, according to comScore. It still lags behind Google (932 million), which will likely be the first Web company to reach 1 billion monthly users, as well as Microsoft (789 million) and Yahoo (622 million). However Facebook's estimated annual growth rate - 73 percent - eclipses all of those sites. (Since comScore captures visitors who are not registered, its estimates skew higher than Facebook's internal numbers.)