The ready reproducibility of not only Facebooks "look and feel" but also its functionality is a direct consequence of the light weight technology that the site is built on. Any technologist knows that Facebook is a really well organized web site, the "well organized" part coming from using the built in hierarchical management of xml to collate messages from to users, to add entries to feeds to transform those xml files into rendered display elements for the site, it is quite a basic xml based publishing platform and nothing more. As such building a similar platform for anyone that knows a bit of xml , xsl and html and jsp is trivial. The "apps" that Facebook loves touting are more managed scripts cleverly categorized but nothing about that is patentable and is easily replicated so Facebook must resort to lawsuits to attempt to maintain its distinct advantage compared to other social networks, in the US where it was founded this works very well but internationally Facebook is pissing into the wind on this one. The local governments have no incentive to shutter domestic clones just so the traffic and ad revenue flies across the ocean to America.
I see Facebooks problem of "wack a clone" as one that many "I coded it in two weeks and I have no strong patents" business models run into, followed by scalability issues. No surprise really, if your design stage is a fraction of your implementation stage you will make mistakes during implemenation and those mistakes will go to production. This will of course necessitate costly re architecting, down time (twitter anyone?) and possible irreparable damage to your businesses reputation.
As I've said before the entire edifice of the current web 2.0 startup fever of getting the idea out and iterating it in production is inefficient. If you can protect your idea with impossibly high barriers to entry like true technological innovation that can not be easily copied before you launch and iterate, then you should take the time to do so after you have that power your ability to fend of competitors will have both a legal and a technological advantage behind it.