"Sorry, I only use Signal" has worked nearly a decade for me
the only people that think it is bad are people who have a different opinion and feel attacked for whatever reason. I find it telling when people accuse others of virtue signaling because it is almost always someone who is jealous or insecure attacking said signaler.
Driving an economically efficient car -- choosing any sort of car -- has enormous consequences on one's life, for example. Choosing to by a particular car isn't a decision made lightly. But Prius drivers back in the day were accused of virtue signaling, as though the Prius were equivalent to a temporary tattoo.
In fact, speaking of temporary tattoos, simply having a bumper sticker advocating for animal rights, say, belief in anthropogenic climate change, or peace in the Middle East will expose one to regular displays of hostility and aggression, so it isn't a cheap signal.
In other words, in my experience your observation is spot on.
Virtue signaling means sending deliberate signals about your virtues, whether you "walk the walk" or not. People are often critiqued for going to uncomfortable lengths to signal their virtues, but something as simple as a "meat is murder" shirt or a MAGA hat is also virtue signaling.
If you change and abandon your principles were they really principles in the first place?
If there is a communications channel by which they can give you their phone number, you can use that same channel to discuss what messenger to use.
And, in doing so, achieve the security posture of the worse of the apps!
If you don't need to? You tell them to get Signal.