Facebook squeaks onto the Fortune 500

• Facebook joins the Fortune 500 due to its rising revenue
• Social network continues to see revenue gains as number of users grows
• Facebook still remains far smaller than technology rivals like Apple

Facebook, the No. 1 social media company, just joined the Fortune 500 for the first time, according to this year's list out Monday.

The annual list of companies, which ranks them by their 2012 revenue, is a way for investors to gauge the relative size of the operations of companies. Fortune magazine publishes the widely followed list once a year.

Facebook entered the list at 482nd with revenue of $5.1 billion, coming in just after cellphone carrier MetroPCS being bought by T-Mobile and just ahead of utility Pepco Holdings. Facebook is just one of 24 companies that were newly added to the list this year.

The list, though, also serves as a big reminder of how relatively small Facebook is given the inordinate amount of attention it gets.

The Fortune 500 is again topped by retailer Wal-Mart Stores with revenue of $469.2 billion, recapturing the spot held last year by Exxon Mobil, which is second on this year's list, with revenue of $449.9 billion and Chevron, at No. 3, with revenue of $233.9 billion.

Another big riser is Apple, the maker of gadgets, which hit No. 6 on the list, thanks to its revenue of $156.5 billion. Were the rankings calculated by profits rather than revenue, Apple would have been second on this year's list, courtesy of the high prices it charges for its products.

There aren't many companies with young CEOs on the list, which makes Facebook somewhat unusual. Just two CEOs of companies are under 40, including Facebook CEO Mark Zuckerberg and Yahoo's Marissa Mayer, who are 28 and 37 years old respectively. The oldest CEO with a company on the list is Dole Food's David Murdock, age 90.