I'm spitballing here, but what if we created a browser extension that gives users the option to block various monopolistic/oligarchic tech platforms. If the browser navigates to a blocked platform, we show a "consider these alternatives" page instead. Competitors to the monopolies could offer incentives (raffles, discounts, etc) in exchange for promotion on the recommended alternatives list, creating an incentive to participate in a boycott.
Example: a user opts into an Amazon boycott, but later clicks an affiliate link that leads them to a book listing on amazon.com. The browser extension intercepts this and displays a list of other places to shop for that book. Barnes and Noble offers a 10% discount for people who boycott Amazon, so they're the top recommended alternative, and the user gets the book for a discount.