If you include traders and creators in the various ecosystems and not just "customer" in the retail sense, examples aren't hard to find.
Google and Facebook. The two biggest ad auction oligopolies own virtually all of ad-space between them. Is there real competition? No, there isn't.
Apple. More and more locked down, less and less reliable, more and more expensive. Want to move? Want to repair something out of warranty? That's going to be... difficult.
Ebay + PayPal. If one doesn't screw on fees, the other will screw you on fees. Ideally both will screw you on fees. Now with added Etsy, because also.
Amazon. The giant changes its mind because something something ToS violation and leaves you with n figures of dead inventory and a locked pay out. What are you going to do?
Spotify. You pay us for the music, we don't pay the artists. [1]
YouTube. The record companies get income even if they don't have the rights to it, for the bargain price of a robo copyright claim. Because recognition algos are so very complicated and they make such terrible mistakes, so much of the time.
These aren't just monopolies, they're massive market distortions where money is so dense it warps the Internet into stagnant pools of pseudo-inevitability, with a chilling effect on real innovation and creativity.
And that's not even getting into issues of privacy, political influence, and the market value of personal data.
[1] Not technically true, but considering the pitiful pay outs it might as well be.