It might be a question of how much money they would lose by not doing it. SOPA enforcement would be insanely expensive for a site with as much user generated/linked content as Facebook.
It's not easy to simply predict how much they would lose if SOPA passed, or even how effective a blackout would be. Personally, I don't think a blackout would have much effect at all. So what if millions of people flood the phone lines of their representatives? Do you think politicians support SOPA because they incorrectly think their constituents like it? That's ludicrous. As for the threat of the next election cycle, most American voters will have forgotten about it by then, and I doubt many representatives have much fear of losing the next election anyway.
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 is EXACTLY why Google and the other big guys are attempting to prevent SOPA from passing. Many people, even on HN, believe that these large entities are on our side, or on the side of freedom, or whatever. The truth is that they don't care about us. They care only about their wallets. All of the time, money and energy they are expending, now, to fight SOPA, is a tiny fraction of what they will be required to spend, in terms of enforcement, should SOPA become law.
