>In what sense are they shitty?

Widespread algorithmic control and corporate spam spreads architecture?

>They are actually usable by normal people.

Usability is just one dimension. If your site is perfectly usable, yet steals your credit card number and sells it to the highest bidder, it is still a shitty site, just not in usability.

>This is a big reason they are popular.

Really FB/Meta became popular long before they were at their current level of shittiness. Really FB wrote the modern book on new levels of shittiness like selling social graphs and tracking people along with connecting all kinds of other data and stealing information from your phone with overly huge permissions and data gathering.