Nick Cooney who says that he's been reading studies that about 25% to 50% of people who say they are vegetarian actually are, though I don't yet have the citations. Thus, if we find out that an advertisement creates two meat reducers, we'd scale that down to one reducer if we're expecting a 50% desirability bias

This doesn't follow. The intervention is increasing the desirability bias, so the portion of purported vegetarians who are actually vegetarian is likely to change, in the direction of a lower proportion of true vegetarianism. It's plausible that 90%+ of the marginal purported vegetarians are bogus. Consider ethics and philosophy professors, who are significantly more likely to profess that eating meat is wrong:

There is no statistically detectable difference between the ethicists and either group of non-ethicists. (The difference between non-ethicists philosophers and the comparison professors was significant to marginal, depending on the test.)

Conclusion? Ethicists condemn meat-eating more than the other groups, but actually eat meat at about the same rate. Perhaps also, they're more likely to misrepresent their meat-eating practices (on the meals-per-week question and at philosophy functions) than the other groups.

A different frame: the claim here is that facebook ads for vegetarianism are unbelievably effective. We can decompose supporting arguments for that into "facebook ads are unbelievably effective" and "vegetarianism is incredibly easy to proselytize."

For comparison, estimates from randomized trials of get-out-the-vote campaigns (where one can actually measure changes in turnout, as votes are counted) are in the tens to hundreds of dollars per marginal voter turned out (before adjustments for other biases, etc (quotes below)).

Some other differences between vegetarianism and voting:

• There is a much stronger moral consensus about voting than vegetarianism
• Vegetarianism is a sustained costly effort, whereas voting is a one-time event
• There are more GOTV campaigns, so vegetarian ads may face lower-hanging fruit
• Images of animals may or may not be more effective than GOTV reminders/arguments

One handy reference is Donald Green and Alan Gerber's Get Out the Vote, which reviews dozens of experiments bearing on the cost-effectiveness of get-out-the-vote (GOTV) efforts.

The key results are summarized in a table on page 139 (viewable on the Google Books preview linked). The strongest well-confirmed effect is for door-to-door GOTV drives, which average 14 voters contacted to induce one vote (plus spillover effects), with a cost per vote of $29 (including spillover effects) assuming that staff time costs $16/hour for staff. Phone banks require more contacts per vote, but are cheaper per contact, with Green and Gerber estimating the cost per vote at $38 for campaign volunteer callers, and $90 for untrained commercial callers.

In recent years, the U.S. political parties have adjusted their GOTV strategy in line with these experiments, and turnout has increased. For instance, in 2004 Green and Gerber predicted predicted that the parties would increase GOTV spending by some $200 million using methods averaging $50 per vote, for an increase in turnout of 4 million, and the turnout data seems consistent with that. This money was concentrated in swing states, and in 2004 turnout increased 9% to 63% in the twelve most competitive states, while increasing 2% to 53% in the twelve least competitive states (while clearly leaving many potential voters home).

ETA:

Cattle have a bit less than 1/3rd the brain mass of humans, chickens hundreds of times less, and fish down more than an additional order of magnitude relative to body size (moreso by cortex). If you weight expected value by neurons, which is made plausible by thinking about things like split-brain patients and local computations in nervous systems, that will drastically change the picture and reduce cost-effectiveness.

Personally, I would care more about a day's experience for a cow than for a small feed fish with orders of magnitude less neural capacity.