I suspect the most important thing for advertising Facebook at this point is ubiquitous visibility - if there is an FB ad, a share button, or a link to a profil

I suspect the most important thing for advertising Facebook at this point is ubiquitous visibility - if there is an FB ad, a share button, or a link to a profile on every page a user visits, it ensures that it is in their subconscious whenever they're browsing, making it that much more likely for them to log in to post or message a fleeting thought that would otherwise have passed forgotten. Similarly, non-users might be momentarily tempted by anything that brings to mind that friend they've not spoken with in years, or that interesting fan group that only has an FB profile, and having links to Facebook omnipresent across the web makes joining that much more likely by reducing the barrier to entry. > ...most people know it's garbage This is likely an exaggeration. While it may be true of the majority of HN readers, HN has a very specific demographic of generally tech- and business-aware people, who are much more exposed to damning data and discussion on the costs of FB - for most users, Facebook is a service somewhere on the scale of "useful" to "essential", with no readily apparent drawbacks; The rather negligible effect of the Cambridge Analytica media outcry serves to demonstrate the average valuation many put on privacy versus convenience. > ...there's 7 billion or so people on the planet, and FB already has 28% of the total population using their app. They surely can't believe that they're one day going to have much more than their current market share? At this point, I'm somewhat surprised FB hasn't started investing in Next Billion technologies; there's a limit to the portion of current smartphone users that are willing to join Facebook, so one solution may be to ensure more people have smartphone access.