"So what? What are people going to do, another "mass exodus"?"No, that's not what I'm saying. What I'm saying is that pissing people off and/or confusing them is putting them into a mindset in which they're not going to be very receptive to the message. The goal should be to get users to care about the SOPA issue on their own volition -- not to beat them over the head with it, especially when it's likely than >90% of them have no idea what it is to begin with.
My point isn't that pissing off the users would be dangerous for Facebook's longterm health. Rather, it's that pissing off the users is unnecessary -- and probably counterproductive -- in the mission to get them to care about SOPA.
"People always get mad at the person who inconveniences them first (in this case, facebook) and then when they see the first one won't move then they get mad at the person who caused that (in this case, SOPA)."
It's a big leap from A to B here, and I'm not so sure your average user will make it. More likely than not, a typical FB user will get mad at FB for inconveniencing him, but will stop investigating at that point. He'll just be mad at FB for a service disruption, and he won't really care about why it's happening. He will be less likely to care about SOPA, because he's pissed at the company trying to get him to care about it.
If Facebook were aiming to maximize receipt and uptake of the anti-SOPA message, then as the saying goes, they'd catch more flies with honey than with shit. Badgering your users into reading about a political stance is bound to be less effective than encouraging them to read about it. Second -- and even more important -- a service blackout does not allow users to do anything about SOPA, even if they cared. All that most of them will do is sit there and fume while waiting for service to be restored (it's a big stretch to think that the typical user will actually read through literature on the subject and then file or sign petitions, or write congressmen, or what have you). A grassroots strategy, on the other hand, allows them to spread the word.
It's not a perfect analogy, but consider the case of People for the Ethical Treatment of Animals (PETA). Back in the old days, PETA activists used to show up outside of shopping boutiques and throw cans of red paint onto consumers wearing fur or leather goods. It got attention, and it certainly pissed people off. But it backfired: people got pissed off at PETA. Nobody made the logical leap from "I'm pissed at these hippies who just threw paint on my $3,000 coat" to "Well, they have a point; I guess I'm really pissed off at the fur trade." Eventually, PETA wised up to the folly of this tactic, and they adopted a new one: getting high-profile actresses to pose nude in tasteful, but provocative print ads. Needless to say, this concept received more attention, and the attention it received was a lot more positive.