Google might be well served by some sort of partial blackout on its home page, but Facebook would need to get more creative -- and, dare I say it, more social.
A blackout of any kind on Facebook would create a minor to major annoyance for an audience that we can presume is largely ignorant to apathetic about SOPA. If, instead, Facebook released profile widget of some sort -- say, a standardized black box as one's default profile image, with a solidarity message or link -- they'd probably see better uptake. People would be curious about why their friends were blacking out their profile pictures, and they'd click to find out more. Even if for no other reason than to see what the fuss was about, or to jump on a trendy bandwagon. In this way, Facebook would be piquing interest, rather than creating confusion.
Anecdotally, I've seen friends use this tactic to great effect when promoting anything from parties, to launches, to causes. It works because it spreads easily, it's hard not to notice, and it's less easily ignored or tuned out than generic site-wide messaging. And, as the folks at Facebook know all too well, we tend to notice what our friends are doing more readily than we notice what disembodied corporate messages are saying.