[This] allows you to make Facebook API calls from any web site and makes it easy to create Ajax Facebook applications. Since the library does not require any server-side code on your server, you can now create a Facebook application that can be hosted on any web site that serves static HTML without any server site scripting.
With this new library, the number of sites, and site owners, that can deploy Facebook applications just increased dramatically. All that remains is for someone to write a turn-key Facebook application creator, as Ning has already done for it's own hosted social networks, and we can expect to see Facebook widgets rapidly proliferating across the Web.
See also: Robert Scoble's interview with Facebook co-founder Mark Zuckerberg