I'm not sure of the resolution at which FB stores photos. At any rate I don't think it's standard practice for people to upload originals in the first place. So large scale printing might not be that simple.
Max they store is 2048px on a side, which is plenty for a 4x6 print. That doesn't mean that all of Facebook's photos are stored at this resolution, as I believe the max size was increased relatively recently.
Home photo albums usually hold 4"x6" photos, if I remember correctly. Most of the pictures in Facebook's lightbox view, are displayed above this on my 15" MBP without a problem. Should be ok resolution wise