Yes, such rules would make companies focus on specific local market first and expand slowly. I'm not so sure this is a bad thing. As you said, it doesn't make a difference to existing big companies. Slower growth for new ones could make it more likely we end up with different competitors in different markets (fb west, vk russia, ...) instead a single winner taking it all. And if some potential ideas won't make it cause of slower growth and a some more administrative overhead, I still prefer that over some large foreign evil empire pushing its social norms onto my country.

Few people already could "enter the market" given the enormous resource requirements to compete with Facebook on it's own. That's not a regulatory thing; that's just a reflection of the reality that these large companies have massive amounts of resources, and similar amounts are needed to effectively compete.

I think we may be talking past eachother. No one can beat FB directly, with a half-decent chance of success. Lots could, with a very low chance of success.

If lots of people can "enter the market" the one of these might succeed. FB is just an example, but I also mean them. Most of these potential fb-killers don't know that they're competitive with FB.

Think of WhatsApp (again, just an example). They entered the market, at an angle. Within a few years, they threatened FB enough to get bought, just to eliminate the threat.

The end result isn't all that heartening, but everything up to that is. I'm not saying GDPR makes this impossible, just worried about the accumulation of these things. Even a handful of reasonable rules could make things harder, especially if they are different in every country. WhatsApp might have decided to focus on a few core markets, and limited their idea to more local things.