Converting website visitors to registered members

First off, I have a feeling that you're misusing the term conversion because it means a prospect's advancement down the sales funnel. Thus, if you don't need people to be members in order to make money off them, then you aren't having a problem with conversion.

Secondly, it's hard to say what the cause is and how to increase sign ups. Most likely either your service doesn't seem to be worth signing up (I'm talking about long-term value not BS one-time incentives like others have suggested) or the site has UX issues (unconvincing copy, unclear calls-to-action, layouts, information architecture, workflows, etc).

If you make money on any visitors regardless of their membership status, you should be tracking retention (repeat visitors) and referrals (traffic from 3rd-party sources and direct hits). Healthy growth of these numbers will indicate word-of-mouth support for your idea.

If you actually need people to sign up to make money, then you've got a long road ahead analyzing visitor paths around the site to identify conversion choke points. You'll need to do a lot of A/B and MV testing to optimize the sales funnel.

However, you still need to talk to your users about their needs regardless of how you make money off them. Low signups may be simply lack of trust for Facebook & your service.

P.S. Definitely get rid of the separate username field. Email address is good enough to be a unique user ID.

Login

I'd suggest OpenID login the way this site and other Stackexchange sites use. Even if the actual authentication mechanism/protocols are different behind the scenes, make the UI consistent. There are several OpenID libraries there to facilitate integration.

Incentives

You should see if you can offer enough WITHOUT signup. This incentivizes the user to actually sign up for tailored service. eg: Anonymous listing of restaurants before login or even a video showing what's behind the login curtains. That alone should motivate users to complete an already light signup page.

Lighter signup:

• You can skip the zipcode requirement or move it to the "complete your profile" page but using their location roughly based on their IP address.
• Try using their email address as the username itself. One less thing to fill, one less item for them to remember. Just remember you should think about people DO change their email address, so your underlying 'username' code should allow for that.

JonnyBoats is right. You need to provide something to people who are signed up that everyone else doesn't get. Perhaps that's the ability to contribute content, as you suggest. Or a weekly newsletter as JonnyBoats said. If there's enough value, visitors will fill out very long forms to get registered. If there's no value, they won't fill out anything.

As far as what information is the bare minimum to ask for, yes, you could probably reduce it. I don't see a need to necessarily ask for more than an email address and a password. The rest could be filled in on their profile page later if they want. The email address allows you to verify that they're a real person, and also can function as their sign-in name. In fact, logging in with an email address tends to be easier than remembering a different user name or display name for every site, in my personal experience. While I don't think it's needed, I can understand why you might want first/last name, but what's the motivation for asking for their zip code? Is this some sort of location based website?

Bottom line is, you need to provide value or you won't get any sign ups. But yes, there's room to reduce the number of fields you ask the user to enter to register, if you think that might be contributing to the problem.

You need to put yourself in the customers shoes, if you are a visitor, what benefit do you get by signing up?

If there is no benefit, then asking for any information is asking for too much.

Perhaps you should focus on finding some incentive to offer people who sign up. Could you perhaps offer discount coupons to some restaurants? Of perhaps a chance to be in the "first 500" hall of fame? whatever, figure out some incentive.

Do you have an email newsletter that people can signup to receive? If not, could you write something simple once a week talking about new restaurants that have opened or that you visited for the first time?

When you delver value, people will signup.