When Facebook sent out an invitation to a mysterious event to "Come and see what we're building," investors took it as a sign that the company will introduce another money-making product.
Bolstered by a series of upbeat analyst reports, the stock continued its climb, adding 9 percent between when the invitation went out and the end of trading Friday, when it hit a six month high. The stock has pulled back nearly 2 percent Monday.
Sentiment seems to be shifting in Facebook's favor; a slew of analyst reports have upped estimates and upgraded the stock. (Read More: One Thing We Know Facebook Is Working On.)
On Monday, Deutsche Bank upgraded the stock to "buy" from "hold" and raised its price target to $40, saying "mobile newsfeed ads are the game changer." And Oppenheimer, which rates Facebook "outperform" raised its estimates and upped its price target to $34. Late Sunday, Piper Jaffray issued an upbeat note, writing "we believe the company will post a strong Q4, in-line with Street expectations for an acceleration to 34 percent year over year growth." (Read More: Is Facebook's Ride Over?)
So what can we expect to hear from Facebook on Tuesday?
Not a phone
Facebook has not invited camera crews into the event, and there is no indication of any partner company. The company has also given no indication that it'll live-stream the event, as it has many prior announcements.
If Facebook were to announce a phone, I would expect it to allude to some sort of partner company involved. And the announcement of a phone would be a big enough deal that Facebook would likely invite in camera crews AND stream the event live.
Not a building
CEO Mark Zuckerberg loves to talk about the products that he and Facebook engineers are "building." I don't think that "building" refers to literally bricks and mortar.
Facebook announced back in August that Frank Gehery will design an engineering office for 3,400 employees that will connect with its Menlo Park offices, which were formerly owned by Sun Microsystem. Zuckerberg explained these plans in a Facebook post.
There's not much more Facebook needs to build.
Something mobile related
The announcement may not be a phone handset, but it's likely to be something involving mobile use of Facebook.
Zuckerberg and COO Sheryl Sandberg have spoken extensively about how important a priority "mobile" is for Facebook, as mobile user growth outpaces growth of the platform on the desktop. Sandberg told CNBC last fall that every product Facebook develops is "mobile first." (Read More: Where Facebook Is Looking to Grow: COO Sheryl Sandberg.)
We could see a new Facebook operating system for mobile devices, so the announcement could showcase a Facebook OS running on a number of devices. With so much of Facebook's growth coming from emerging markets, where high end iPhones and Android handsets are more rare, this could be designed to enable Facebook and its various apps to work better on lower-end phones.
Facebook's growing mobile use includes tablets, so Facebook could extend its 'Messenger' app, to the iPad and android tablets. Eventually we should look for Facebook to introduce video chat—like Apple's"Facetime"—for tablets and smartphones. But if it's a tablet-related announcement, it would likely expand what Facebook offers for smartphones—group messaging and photo sharing, as well as the voice message feature—to tablets.
Something Search Related
Zuckerberg and Sandberg have both discussed the massive potential in search. The next step Facebook would announce in this arena is a better way to search within Facebook. Then eventually we'll see ways to use a Facebook network to deliver targeted search results from across the web.
Topeka Capital markets estimates that Facebook would generate "close to $5 billion in annual revenues and $2.5 billion in EBITDA from a social search engine." I don't think the search engine will be revealed Tuesday, but I would expect more news on it in the next few months.
(Read More: Here's What Facebook May Unveil.)
Ad Changes and Expansion
Facebook has been working on a number of advancements for its ad business, including video-ads and an ad network, to deliver targeted ads to sites across the web. We could hear about better rotation of ads, so people aren't bored by the same ads over and over.
Facebook could change its timeline format, to make ads more carefully integrated and less intrusive. Or we could hear improvements to its Facebook Exchange—a real-time bidding ad system for advertisers to target users.
What if it's no big deal?
It's possible that the media frenzy over this announcement leads to a bit of a let-down.
Piper Jaffray's Gene Munster wrote, "We believe that despite speculations around a major announcement, it is more likely that the company introduces modest upgrades to the user and/or advertiser experience. If shares pullback as a result of investor disappointment, we would be buyers on the pullback."
No matter what category the announcement falls into, I would be surprised if there weren't revenue implications down the line. Zuckerberg made it clear in its last earnings announcement that he's serious about turning Facebook's popularity into profits.
(Read More: Video Ads May Be Coming to Your Facebook Feed.)
—By CNBC's Julia Boorstin; Follow her on Twitter: @JBoorstin