Facebook has built something, its cryptic invite for today's event informs us. What exactly that something is, well, we won't know until 1 p.m. Eastern. The social network has kept this under the "masterful" shroud of secrecy. But that doesn't mean we don't have educated guesses. Stay tuned for live updates, but here's a reality check on the three big rumors while you wait:

A Facebook Phone

The Rumor Mill: That key word — "building" — has a lot of people talking hardware in some form or another, which has the Facebook phone whispers picking up again. TechCrunch's MG Siegler cites "multiple sources" telling him to expect "some sort of Facebook phone to be on display Tuesday." But even Siegler doubts Facebook will unveil its own hardware, with a more likely scenario being a Facebook partnership with a gadget maker, such as HTC, while the social network provides the software. Facebook already has (kind of) starting building its own operating system with all the apps it has announced of late, as ABC's Joanna Stern points out. It wouldn't be too hard for Mark Zuckerberg and Co. to take Messages, Camera, Instagram, Poke, and other Facebook apps, then roll them into one OS with a couple new features. (Like, you know, Phone.) 

Actual Likelihood: Unlikely. For a brick-and-mortar (and aluminum) phone, that is. But (!) some type of software is totally possible. Zuckerberg has denied that Facebook will ever build its own phone on many different occasions, including just this past September in his first major interview after taking the company public. Facebook has also reportedly told partners like Apple and Google that it has no intention of jumping into the phone-making game. All of which makes a Facebook phone sound pretty impossible. But, considering the company's borderline desperate push toward mobile, a Facebook OS sounds a lot more plausible. 

A Social Search Engine

The Rumor Mill: Facebook has what Google wants — it knows what your friends like, at least theoretically. And nobody really uses Google's social search because Google+ doesn't actually represent one's social circle. But Facebook pretty much does, and it could build a personalized search product based on all of its billion-plus users and all of the their tastes therein, from music and movies to potential advertisers and interaction histories. 

Actual Likelihood: High. Analysts say a search move makes a lot of sense for Facebook, and Zuckerberg has talked how the company is "uniquely positioned to answer the questions people have," according to an interview with Michael Arrington at the Disrupt conference last September. This isn't just a theory for Facebook, either — they have "aggressively" expanded their search staff to 100 people, according to The New York Times's Somini Sengupta. And analysts predict Facebook could make a lot of advertising money from search, and Facebook cares about more and more making money off its products these days. 

A Site Redesign

The Rumor Mill: Face it — Facebook.com has a lot of areas that could use aesthetic improvements, but it also has utility problems. If Facebook goes for a redesign, it would likely make things, well, easier to find, either on the Timeline, as TechCrunch's John Constine suggests, or in the News Feed. 

Actual Likelihood: Very possible, but not big enough. If Facebook went out of its way for all the Apple-style event hype for just a refresh that it typically rolls out with a note on the site from Zuckerberg, it's got to be part of a larger rollout — possibly in connection with the social-search product. But the current Facebook design does it difficult to sift through all that information, and making search easier would fix a lot of that. Facebook's search might not end up a standalone search engine like Google so much a new product baked into the existing site, which would probably have to involve a redesign of sorts. 

Stay tuned right here for full coverage at 1 p.m. Eastern.