If you are an Open Source Developer or Content creator (If you are a member of either service you are a content creator) you should not use Facebook.

By using Facebook you are essentially raising the value of their company and their application Facebook is a closed source content silos that does not allow you to control the content that you create.

Facebook does not put the content creator/members at the top of their pyramids when thinking about revenue models. Facebook puts their Facebook first above the members and communities that have given them value and revenue.

If you are a developer you may be able to make some money by creating applications for Facebook ; but I do not believe that Facebook will ever allow your application to eat into their user base or their revenue. Because Facebook is a closed source companies that have the ability to literally cut you off by changing the code/api or by using their proprietary knowledge to build an application that you can not possibly compete with.

As a coder understand that when you build and extend Facebooks propitiatory platforms that you undermine the longevity of the Open Internet.

Content owners and Developers do not help closed source companies (Facebook) in their goal of creating another closed source content trap that will extract hundreds of Millions on dollars from their member and developer communities and give nothing back in return.

“Much like how one FBML tag is a shorthand way to include a lot of markup (think of fb:comments, for example), one custom tag can replace a lot of FBML and HTML content inline. Using custom tags in this way lowers the communication overhead between Facebook and your application’s servers. One custom tag can render a larger amount of content. Instead of sending the full FBML fragment with every request, you can replace it with a custom tag that Facebook will expand when Facebook parses the page.”

As a FB app developer I have absolutely no idea how that could possibly be true. Sure, the markup might save a few bytes (and server side) processing, but whomever serves up the custom FBML will still push their own markup to replace what you've saved and their servers will process the cycles your servers would've handled. If anything, the additional load from a third-party provider would INCREASE load times (though probably minimally).