Facebook Connect is a new(ish) service from Facebook that allows integration between a users Facebook account and a third party website. The basic idea being that if a user updates their profile on Facebook the data gets updated on the third party website.

Basics

To start there are 4 different areas of integration with Facebook:

Identity. Seamlessly connect the user’s Facebook account and information with your site
Friends. Bring a user’s Facebook friends into your site. You can link friends that have existing accounts and even invite new friends to your site.
Discovery. Publish information back into their friends’ News Feeds on Facebook.
Privacy. Bring dynamic privacy to your site using XFBML.

Using the above description, it appears that integrating FriendFeed Connect on your site, especially a social site, allows users to keep their online profiles in sync between websites.

For example:
Assume I have a social site with Friends, profile pages and privacy settings.

When a Facebook user logs into my site I can prepopulate their profile data on my site with their profile data on Facebook. If there are other users on my site with a Facebook account I can move the users Friend list over as well. Finally, if my site allows customization of certain privacy settings I can keep those in sync as well.

Additionally, activity done on my site by a Facebook user can be pushed out to their Facebook profile.

And the coolest part? The user just updates their Facebook page to update my site!

Pretty cool really.

Not all sweet though…

There are some interface destroying guidelines, as well as some restrictions, third party sites have to abide by.

1. The website must display and use one of the approved Facebook Connect buttons on Facebook Connect Login Buttons to begin the “Connect” process.

2. When Facebook Connect is used to allow a user to “sign in” or authenticate with your website, the Facebook Connect option must be presented at least as prominently as the most prominent of any other sign in or authentication method on your site, and not as a secondary option.

4. When a user has connected their account and is currently logged in on your site, the application must show the profile picture and name of the user in a visible place on the page. This profile picture should be no smaller than 16x16 pixels and should include the Facebook “f” favicon in the lower right corner. If the profile picture is too small to position the icon over the lower right corner, you can position the favicon to the right of the picture.

6. If you have an application on Facebook and a website that implements Facebook Connect, the user will need to explicitly connect on your website before your website can use their information. However, once the user has both authorized the application and connected to your website, you can link the information.

8. Facebook Connect applications cannot publish one line stories automatically via the Facebook Platform API (i.e., without being presented to the user in a Feed form) unless the story and template have been approved through the Facebook Connect approval process.

9. When a user completes an action in an application that would prompt a Feed form, the application should add a check box (to the part of the flow associated with completing the action) asking the user if they want to share this information through Facebook (e.g., “Share this run with my Facebook friends?”).  The check box may be pre-checked by default, but if the user unchecks this check box during the flow, the Feed form should not be shown.

11. When representing a list of friends to a user on the website, which includes a set of friends from the user’s full Facebook friend list, the website must indicate that the friend relationship on this website is associated with their friend relationship on Facebook. It is recommended that you use a Facebook favicon or the word “Facebook” to indicate that these friend relationships are associated with Facebook.

Still, even with all the work the above entails, it’s a compelling service.