Are we really going to assume that your average user doesn't already have FB installed and already signed in? They have over one billion users, you know. It's not exactly rare to find Facebook on any arbitrary person's device.
Even assuming all of those accounts are still active (and none of them are duplicates), that still leaves around 7 billion people without a Facebook account. Personally, I do have a Facebook account, but I only use it on my private desktop PC, not on my work laptop and also not on my phone.
I agree with you, I'm only logged into my work FB account on my personal device, however I'm also a power user (and I'm guessing you are too) so I don't think our experiences can extrapolate out to the population as a whole. Sure 7B people don't have FB, but are you trying to invite 7B people to your event or business? Not really; you're probably trying to target a very specific kind of person, or a group of people you already know. In that case you can just do some surveying / focus group'ing to figure out what people already use and push your content on the platforms that cover the majority of users (which in most cases, I'm guessing, would include FB).
