How much is a heap of sand? It's a Sorites problem. There exists a continuum between "changing your profile picture to be tinted like flag X" and "everyone is saying and doing the same thing and it's an entrenched social norm".
It's certainly less impactful than doing something substantive but it also costs nothing to signal boost. Same like boycotts, it only works en masse.
1) Does "showing support," actually do anything? Are we really aligning on norms or just scoring points with people who already agree the same position? I suspect the detail matter and that there is continuum, where for uncommon positions maybe it does something, but for widely held views, it really is just "virtue signalling."
2) When does "showing support," become a substitute for more substantive action. Maybe I post a pride flag on my social media avatar, but don't bother to vote in a local election with discriminatory ballot initiative. Or consider any number of incidents of corporate "greenwashing."
But sure, plenty of virtue signalling, isn't _just_ signalling. And we shouldn't dismiss it on those terms, but rather ask about impact.