How easy it is to make people believe something they want to believe... Inevitably, that's something all "information campaigns" must consider, whether their information is commercial propaganda or ideological activism. If someone is uncomfortable with the (totally true) accusations against McDonalds, they can just choose to believe the hamburger chains counterpropaganda.
When information is spread that McDonalds hamburgers are unhealthy, McDonalds doesn't have to deny them; they just have to say the opposite thing the next day. Then people who don't want to "get involved" can cite McDonalds' shameless lies.
My message is, information campaigns are overrated. Information does not convince people, people convince people. In the long run, daring to voice one's opinions in public is the best way to counter McDonalds (this may sound like it's self explanatory, but I've often seen people with strong opinions keep them to themselves when they're in the miniority, or percieve themselves to be) is to give your opinions, loud and clear, and with your name all over them. Information campaigns can be countered, but unless McDonalds bribes people to organize some sort of countermovement, they can't counter it.
Paying people to organize themselves for a shifty cause has actually been done by megacorporations, I've read reports about "smoker's rights" movements in the USA being founded by the tobacco industry. Anyway, such acts are so contemptible that I think few corporations would try unless they have a really bad reputation already. McDonalds is trying to build a kind reputation, so they're wide open. Not from campaigns but from people.