I never understood that phrase. If you admit there is virtue to your opponents actions isn't that just certifying you are wrong regardless of the opponents intentions?
No, it's saying they're doing something mostly superficial and useless because they think it will make people see them as virtuous, where they wouldn't have done it if it wasn't a highly visible act, not that the actions are actually virtuous. So like someone volunteers for one day for some charitable cause, but spends the whole time taking selfies and not actually helping much.
That said I'm not sure what the logic is that quitting facebook counts as this
Alright but the highly superficial act is seen as virtuous. The act we oppose when we use this phrase. That act. It is virtuous. Therefor we in this hypothetical stand against virtue and goodness.
It generally means that we don't believe they'd be taking that action if there weren't a camera rolling or trending hashtag to follow. It's not criticizing the actual action, but the context around the action.
We're agreeing that the individual act is virtuous. You're not understanding that complaints of virtue signaling are not criticizing the individual act. They're criticizing the unspoken lack of other acts.
An act being seen as virtuous doesn't mean it actually is. Or it could be only a little virtuous, but outweighed by how smug and obnoxious someone is being about it.
"gives more credibility"? Think about what consensuses various cultures through history and currently have arrived at about what is the right thing to do and who is worthy of admiration. Someone who assumes conformity = virtue would end up being pro slavery in most of those, public opinion on morality is wrong most of the time about most things, and pursuing it isn't the right thing to do at all, let alone the same thing as actually trying to be virtuous.