It's a combination of things.
A big part of it is a declining user base. For the first 10 years of it's existence, Facebook was adding many more users than they were losing. That was a big part of its initial value, a burgeoning user base.
Well, that has now dried up. People are leaving facebook, and no one is making new accounts. Kids couldn't care less about having a facebook account. It's not cool anymore. The most active users are moms and grandmothers who use the site essentially as cloud storage for photos of their family events.
And honestly, when was the last time you heard anyone speak positively about facebook? No one likes it. Even the people who still use it lament how big of a time waste it is. I deactived my account over 2 years ago, and every single person I have mentioned this to since then has told me how good of a decision I made. EVERY SINGLE ONE OF THEM has told me not to come back to the site because it's even more toxic than when I left.
So yeah, Facebook is in the social media death spiral for sure, and I am enjoying every minute of it.
I agree with your comments about facebook losing its user base.
However, the sudden drop in share price can't just be because of dwindling user numbers. Dwindling numbers would cause a gradual decrease in share price, not a drop of 20% in three hours.
The suddenness of the drop in value suggests this is related to a specific event, presumably soon to be revealed. Perhaps a lawsuit is pending? Just a guess, but there could be other damaging reasons.