So the reality they must face is that the social media giants will try to circumvent regulation.

Fine. And the most reasonable response to that is therefore to implement some form of general anti-avoidance rule, of the sort that's used in, for example, tax, company, and employment law.

For instance, you could say that if a social media company were to change its behaviour to avoid a law of this sort then they should incur a charge equivalent to, say, 10% of its global revenue. That ought to make compliance with the law the more attractive option.

There's no reason to be defeatist about this - social media companies are not and should not be above the law.