The earliest really large-scale instance of this occurred in 2001, when more than 30,000 researchers signed a letter boycotting any journal which didn't make articles open access within 6 months of publication. The boycott meant not reviewing for, editing for, or publishing in, any such journal. Signatories included many of the best-known scientists in the world. One of the authors of the boycott was Nobel Laureate Harold Varmus, who had just completed a term as Director of the US National Institutes of Health.
It's unclear how many of those researchers followed through on the boycott. Some did -- I've met many scientists who say they send letters similar to LeCun's. I've also met some who signed, but who now admit (often sheepishly) that they didn't follow through.
The donations and open-access merchandise are a nice extra.
They are turning fabulous profits (almost $3 million in 2010; $4 million in 2011 -- these are net profits), but, being a non-profit, must invest them back in the organization.
It's unclear how many of those researchers followed through on the boycott. Some did -- I've met many scientists who say they send letters similar to LeCun's. I've also met some who signed, but who now admit (often sheepishly) that they didn't follow through.
Text of the letter is here:
http://www.plos.org/about/what-is-plos/early-history/