When you turn on the phone, for example, your Facebook home screen may pop up and the phone's camera may directly connect to Facebook services, like Instagram. Sources have hinted at both those features to The New York Times' Bits blog.
Still, it's not clear anyone wants a Facebook phone, and if they did, there's a good chance they wouldn't want it on an HTC device. Other phones with deep Facebook integration have launched before, and they haven't sold well. Earlier, HTC created a device called the Cha Cha which let users post photos instantly to Facebook. It was a bust. Another Android-Facebook phone attempt, the Inq Cloud Touch, was pulled before it went to market.