They are not monopolies. They own a segment of users but NOT a segment of advertiser budget. They compete with each other for their true customers: advertisers

They are not monopolies. They own a segment of users but NOT a segment of advertiser budget. They compete with each other for their true customers: advertisers and their money. If one platform gives a higher ROI than another then more advertiser budget will shift to it. Sure, most large advertisers will diversify but even a 1% shift means billions in this case so worth having people try to optimize.