Typically javascript can be inserted in either the HEAD or the BODY; the latter is usually recommended whenever possible, and ideally as close to the end as possible (just before the </body> tag), so page display doesn't get stalled waiting for the script to load and execute, which can happen when it's in the HEAD or higher up in the BODY.
Specific technicalities of a given script can require it to be loaded in at a certain point in the page, but typically this pertains to positioning within the BODY. I don't know why FB may require theirs to be placed in the HEAD, or whether they merely recommend that for some reason rather than it being a strict technical requirement, but that placement is hardly a universal "coding standard". We'll post a request to our wishlist requesting an option for HEAD placement at some point in the future, but we cannot estimate when or even if that request could be implemented.
