Ah. I haven't used it on desktop in a couple of years. And even then, I wasn't actually "signing in" to it. It still worked just fine, via Facebook's page(s), so why bother, and this also seemed to avoid their announcing your online status.

On mobile, if you don't know how else to reach out to someone, Messenger's your best chance. Closest thing to a modern-day AIM that I've seen, in terms of ubiquity.