Each brand within a portfolio still has a team of people for marketing, sales, etc including analysts that can surely tell if the brand is selling more or less.

Each brand within a portfolio still has a team of people for marketing, sales, etc including analysts that can surely tell if the brand is selling more or less. P&G has a huge portfolio of products but they still have people dedicated to Head & Shoulder, this isn't just bundled up all together where nobody knows what's happening with each product line. As for how many people need to join a boycott to make it worthwhile, well this is the same argument as voting. One at at time might not seem huge but once more people know and change their habits, this does have an effect on the market. See organic food, that did start very small but there has been a slow but gradual switch in consumer behaviors.