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(CNN) -- Foursquare, Gowalla and other mobile check-in services could get some big competition on Wednesday. Really big.

Facebook, the 800-pound gorilla of the social-networking world, has scheduled a media event for Wednesday afternoon at its Palo Alto, California, headquarters. Virtually everybody paying attention predicts the site will announce its long-expected leap into the world of location-based networking. That might allow users to tell friends exactly where they are -- using the GPS on their phones -- in addition to what they're doing and thinking.

A Facebook announcement is scheduled to begin at 7:30 p.m. ET.

For months, Facebook has barely tried to conceal details about its plan to jump into the check-in spac... e, which is dominated by smartphone apps that let users tell their friends where they are at any given time. Facebook all but promised to debut a location-based feature at its F8 Conference in April.

"We are working on location features and product integrations, which we'll be launching in the coming months, and we'll share more details when appropriate," spokesman Larry Yu told CNN content partner CNET last week.

But the tech pundits have speculated incorrectly about Facebook's leap into the world of location features before.

Earlier this year, the tech community anticipated such an announcement both at March's South by Southwest Interactive festival in Austin, Texas, and at the F8 conference in San Francisco, California.

Analysts say that when complaints about user privacy on Facebook prompted changes to the site's settings this spring, that may have slowed its progress on the check-in application.

The past year has seen big growth in the relatively new mobile check-in space.

Foursquare, which essentially made its debut at South by Southwest in 2009, was one of the combatants in a so-called "Location War" at this year's festival, going heads-up with Austin-based rival Gowalla.

Since then, Foursquare has become the clear favorite of the two -- and a darling of tech media, even as it looks for ways to turn a profit. Boasting a half-million users at the start of SXSW, Foursquare doubled that in about a month then needed only another three months to hit 2 million last month.

On mobile apps like Foursquare, users "check in" at restaurants, bars, movie theaters or just about anywhere else they visit. The idea is twofold -- to let friends know what you're up to while racking up points and earning virtual badges in the application's gaming element.

The New York-based Foursquare has begun forging some corporate partnerships and some businesses have started offering discounts and other deals for users who check in frequently.

But Foursquare's relative success could be dwarfed by the sheer volume of Facebook's user base. Its half-billion users represent a massive potential user base.

"If Facebook enters into this, and I don't think there's any reason to think they won't, they'll just slaughter everybody," said Alexandra Samuel, director of the Social and Interactive Media Center at Emily Carr University in Vancouver and a panelist at South by Southwest, told CNN at the time.

But Facebook's platform is expected to allow third-party applications to integrate with it to offer at least some of its check-in functions to developers of new apps, which could offer outsiders more of a chance.

On Tuesday, Foursquare CEO Dennis Crowley seemed nonchalant when asked by a Twitter follower about the potential Facebook announcement.

He wrote: "Ah, check-ins are just 'Act 1' of the @foursquare story. We getting ready to roll out Act 2 and Act 3. :)"