The sleek new Heller Manus-designed glass tower going up downtown next door to Salesforce Tower, known until now as 181 Fremont, could end up becoming Facebook Tower if one real estate rumor proves true. At the very least, it sounds as though the social media giant is considering taking some space in the building for part of its Instagram staff — and as TechCrunch notes, because Facebook has no offices in the city, whenever anyone has to have a meeting in SF they're forced to do it at the offices of Facebook's PR company OutCast.

A source in the company who spoke to TechCrunch downplays the scale of Facebook's SF plans, saying, the company is "considering this a 'test' or 'pilot' of San Francisco office space." And this comes a year after we heard persistent rumors that Facebook employees were begging the company to open up an office in SF because of their awful commutes to and from Menlo Park — something TechCrunch confirms remains an issue for recruiting and retention.

One potential plan apparently calls for bringing "up to 100 Instagram employees" from the office they currently have a mile away from Facebook's main, Frank Gehry-designed headquarters. Instagram currently has about 500 employees.

But The Registry has a different rumor, "according to three sources who track leasing information in San Francisco," suggesting that Facebook may be interested in leasing the building's entire 432,000 square feet of office space — the building's top 17 floors are going to be luxury condos, with the bottom 50-odd floors devoted to office.

That would obviously be way more than a "test" office space for the company, but given how tight the SF office market has been at various points in the recent past, it could behoove the company to be thinking ahead to a time when they will want a significant, high-profile space in the city on contiguous floors.

The Registry also heard from one of the sources that it could be Instagram that's a big piece of the SF space.

Developer Jay Paul is touting the office space as having a unique glass curtain wall system that maximizes natural light, as well as column-free open floor plates, a state-of-the-art water recycling system, "unprecedented views," and advanced technological features.

Next door at Salesforce Tower, previously un-spoken-for floors are being snapped up quick. Salesforce, which had only been committed to taking about half the building's 61 floors, is reportedly talking about taking 200,000 square feet more according to the Business Times. And WeWork is in talks for space in the building as well.

Stay tuned.

Related: Salesforce Tower Officially Tops Off; Top Floor To Be Event Space Available For Community Use