This question is more of a question on whether boycotting can be an action of effective altruism, rather than a particular cause.
Pessimistically, it seems that there is no way to stop factory farming - and that veganism is not really that effective at all. Furthermore, boycotting companies like Nestle does not seem to make any sort of difference into the atrocities they commit.
As an EA, should you not care about boycotting?
I don't know much about the Nestle example, but in principle yes I think so.
I think the same would apply to any case where the production of each individual product does marginal harm. In that case a single individual can choose not to purchase the product and therefore have a marginal impact.
And maybe these kind of boycotts are more common than I suggested in the original answer, but it definitely applies to veganism.