I think I also react very negatively to everyone being on the same dominant social media platform. But for me, I think it is less a concern for privacy, and something more aesthetic. I know this because I don't really get that upset about privacy in other domains, so why do I react so negatively to social media platform dominance?

I think aesthetically, I just don't like the idea of too much conformity. I don't like it when people do something simply because others are doing it. I think it's just ugly. It may be practical, but I just don't like the way it feels. I feel this way for smaller things too... like, I don't like how some people start getting into sports because other people are into sports, or wear a certain style of clothing just because others are doing it. And I sometimes actively rebel against that.

So, yeah... I feel the same way as you, but more as an aesthetic reaction than a real concern for privacy.