but analysts say the social network wants to spread roots across the Android platform.
Facebook's invitation says only: "Come See Our New Home On Android."
Technology news site TechCrunch predicted the announcement would be a modified version of Android with "deep native Facebook functionality" on a phone made by Taiwan's HTC.
Android, the free mobile operating system from Google, accounted for 51.2% of US smartphone sales in the three-month period ending in February, according to freshly-released survey results from Kantar Worldpanel ComTech.
Android added 5.9 percentage points from a year earlier, while Apple's share fell 3.5 points to 43.5%.
Windows Phone, boosted by the new operating system introduced last year from Microsoft, increased its share to 4.1%.
Facebook has made a priority of following its more than one billion members onto smartphones and tablet computers, tailoring services and money-making ads for mobile devices.
"It is really clear from the stats and my own personal intuition that a lot of energy in the ecosystem is going to mobile, not desktop (computers)," Facebook co-founder Mark Zuckerberg said in September.
"That is the future."
Zuckerberg rejected suggestions that Facebook would make its own smartphone, adamant that the company had no intention of stepping into the fiercely competitive handset hardware arena.
"Apple, Google, everyone builds phones -- we are going in the opposite direction," Zuckerberg said at the time.
"We want to build a system deeply integrated in every device people want to use."
HTC forming an alliance with Facebook makes sense since the handset maker could capitalize on the social network's marketing power in an Android arena dominated by Samsung, according to Silicon Valley analyst Rob Enderle.
"It is not so much a Facebook phone as imagined by Zuckerberg as it is a lifeline for HTC, which needs a champion device and got Facebook to back it," Enderle said.
"The market has been gravitating toward Samsung," the analyst continued. "This device is to take the emphasis back to HTC and provide them with a flagship phone that users can get excited about."
If Facebook had been eager to build its own phone, it could have reached out to close partner Microsoft, which owns a small stake in the social network, Enderle reasoned.
A feature of Windows mobile software is integration with Facebook.
HTC is among the electronics companies that make Windows-powered smartphones, increasing likelihood that a Facebook-centric handset features a tie into the software titan's Bing Internet search engine.
A Facebook-focused phone's goal would be to make it intuitive to shop, search, post or do other tasks using the social network's services.
"You will see a phone laid out to address the many ways that you can get into Facebook," Enderle said.
"A main page with a Facebook feel and your news feed nicely displayed, and optimized to allow you to live in Facebook much like you live in iTunes when you are on the iPhone."
If reports are correct, the device will help Facebook more easily connect with mobile users, and -- importantly -- deliver more ads in the fast-growing segment.
The research firm eMarketer said US mobile advertising spending grew 178% last year to $4.11 billion, a market that nearly tripled last year.
Google took more than half of those revenues, but Facebook's share in growing: eMarketer said the social network accounted for 9.5% of mobile ad revenues in 2012 and is expected to take 13.2% this year.
In the mobile display market, however, Facebook is on top, projected to grab nearly $3 in $10 this year, the research firm said.