Are you suggesting they effectively treat the first six weeks of employment as little more than an "extended interview" and fire anybody who doesn't make their cultural fit in that time frame?
FWIW most european countries allow 1-6 months probation periods (for non-temp jobs, temp jobs have either no probation or much shorter ones). During probation, either side can terminate the contract without most of the restrictive clauses applying, it's essentially a short period of "at-will" to check fit between the company and the new employee.
The main reason I view it that way is because Facebook hires a lot of fresh college graduates who don't necessarily have much (if any) money saved up. To work for Facebook, they're going to have to find a place to live in one of the most expensive areas of the United States. I think it is quite disrespectful to put young people with little money in a situation where they are signing extended leases only to be let go within six weeks.
Granted, Facebook gives out pretty good signing bonuses, so that softens the impact a bit, but still if I knew that the first six weeks were effectively a probationary period I'd make a special effort to find a housing situation where I could pay on a monthly basis rather than entering a six- or twelve-month lease.
Moreover, if I had a family, I'd definitely think twice before going into a sort of six week interview, no matter who the company was. I wouldn't want to put my family through the difficulties of moving there only for me to get let go soon after arriving.
If it's not clear that's the case, which seems to have been the case in these anecdotes, it would be pretty unacceptable.
Telling people "Welcome to Facebook," having them quit their old job, and have them relocate just so you can smoke 'em and throw out the trash might be efficient, but it sure isn't kind.