The thing to keep in mind is that SOPA, if I understand correctly, only applies to foreign entities. As it stands, FB won't be impacted substantially by the legislation.
That said, it'd be in their best interest to stop this now, because even if this doesn't impact them, slowly but surely the laws will be amended to apply to everyone.
Facebook, Google, at al have to be ready to pull ads for, links to, and results containing blocked sites as well. They have to continue to monitor that over time as well.
I would imagine a two-phase approach: phase one would be the black-bar-type-doodle logo, phase two would be shutting things down. Whether phase two would happen would depend on how likely the bill is to pass, how well phase one went, and how much it would cost to police if it passed.
Observe that the definition of "foreign Internet site" does not state that the site is located outside the USA, owned by a non US Citizen, or any other commonly understood meaning of the word "foreign". All "domestic" sites also meet the definition of a "foreign Internet site" "for the purpose of this section":
(a) Definition- For purposes of this section, a foreign Internet site or portion thereof is a `foreign infringing site' if--
(1) the Internet site or portion thereof is a U.S.-directed site and is used by users in the United States;
(2) the owner or operator of such Internet site is committing or facilitating the commission of criminal violations punishable under section 2318, 2319, 2319A, 2319B, or 2320, or chapter 90, of title 18, United States Code; and
(3) the Internet site would, by reason of acts described in paragraph (1), be subject to seizure in the United States in an action brought by the Attorney General if such site were a domestic Internet site.
Seeing that kind of doublespeak codified into law is downright frightening. I watched that SOPA markup hearing a while back and all the reps were talking about foreign this and foreign that and how these targets of prosecution were in other countries so most wouldn't even show up in court... yet the definition of foreign is a US-directed site?
That said, it'd be in their best interest to stop this now, because even if this doesn't impact them, slowly but surely the laws will be amended to apply to everyone.