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?
Boycotting animal products is quite effective on its own. Avoiding eating one single egg prevents around 24 hours of chicken suffering in expectation, which is pretty significant. Eating one more chicken causes around 30 days of additional intense chicken suffering. Your boycott would have a very significant impact in expectation.
The problem with animal product boycotts is that we haven't discovered a robustly cost-effective way of getting other people adopt a plant-based diet. My current position on this issue is that: "It's impactful to go plant-based, but it's very hard to get other people go plant-based".