Considering the average annual revenue per U.S. Facebook user is ~$20, startlingly less in other countries, arguably Microsoft, not Facebook, is the more interesting company. Nearly as many active users as Facebook, but substantially more revenue per user (~$50-$150 for an OEM key? Add in office subscriptions, enterprise users, oh my) and substantially more "eye-time" (monthly active? once per month? ha. you used Windows the entire time you were on the PC). Arguably. I don't want to argue it. It's just a thought and it's not the point.
No one looks at Microsoft and thinks "they're going to absolutely dominate the home computing OS market in 2030." They owned the planet and they threw it away. At best they'll remain a second-rate player in the cloud market, riding the ripples of their 90s enterprise clients until those die out as well.
Meanwhile Apple and Google ate their cake. No one saw that coming in 2001. Wait, the company that nearly went bankrupt and now makes that really bad MP3 player, and... Google who? Hold on let me AltaVista that. The companies that are, today, worth almost nothing are worth a combined $2T in just 17 years? What the hell happens to the US Dollar in the next decade?!
Shit changes, and it changes overnight. Sometimes literally, most of the time just "faster than you expect". The only thing you can bet on is that "active users" means about as much as the moon landing conspiracy theories.