I don't have a Facebook account. If Facebook Connect is the only option, as it is for the comments section on The Huffington Post and as it once was for Answers.com and Spotify and Zynga's games, I'll leave and not come back. And there are many other web users like me in this respect. Some are paranoid about Facebook's privacy practices, while others just don't feel like creating a Facebook account just to log into one site if they already use a Twitter account or an OpenID identifier (Google account, LiveJournal, AOL, Yahoo, etc.) for other sites. You might improve conversion by supporting more identity providers, much as Stack Exchange supports Google and other OpenID providers in addition to Facebook.
Even among Facebook users, some people don't want their posts associated with the name on their Facebook account. They might want to keep their work identity and hobby identity separate. Or they might want to provide politically sensitive information but do not want it associated with their real name, in order to avoid retaliation from a state or employer. A lot of sites where Facebook Connect is the only option provide no way to post anonymously or pseudonymously; instead, they always associate each post with the name on the Facebook account.
And some apps require not only a Facebook account but more specifically a verified Facebook account. The comments section on The Huffington Post is one of them. Verifying a Facebook account requires a mobile phone capable of receiving SMS messages and a valid subscription to SMS service on this phone, and its number must be globally unique among Facebook users. This requires paying a recurring fee to a cellular carrier and excludes users who use a land line and users who share a cell phone with another Facebook user who is verified.