There's a couple things to consider (having learned the hardway recently).<p>The company I work for, <a href="http://theinsider.com" rel="nofollow">http://theinsider.com</a>, uses Facebook Connect on our site. Before I started there, they had a fairly basic implementation of FB Connect that was put in place when that program was first made available. About 2 months ago I went through and re-wrote the entire FB Connect implementation our side to bring it up to date because the FB Connect API had changed a bit.<p>Within 2 weeks after we pushed my changes live, FB changed the 0.4.0 API we built against which resulted in some breakage on our side. This was pretty much without notice. So in the past week, I had started going through my code to update it again when I found that there is a completely new FP Connect JS api that is being released.<p>I got excited about it because the new api is much simpler to use, the js library is 10x smaller than the old one(which could pull in at close to 200kb at times), and its much better documented than the old one. However, its still alpha and we're having a couple of minor problems implementing it. That's mainly do to a change in the cookie structure, which is a problem we're addressing on our end.<p>The point of this is that in my experience, FB doesn't document much of their APIs in a consistent or well-organized manner. This is especially true on the FB Connect side of things. They can make sudden changes on their end that adversely affect companies and websites that are using their systems.<p>On the plus side, using FB Connect on our site has been really successful. We're getting more user interaction, more engagement, and higher page views at least in part of utilizing it.<p>The question of whether to use Connect or build a Facebook app isn't either/or. It depends on what kind of experience you're trying to build. The downside of FB apps is that they've been around for a while and have lost their luster. They really just clutter up a user's profile. Its much more difficult for users to find apps on Facebook now than it was a year ago, due to the constant changes that FB is making.