FB's emphasis on family connections makes it much more... potent, and almost cult-like in some ways.
Families stay in touch and plan events on FB. Your mom, aunt, and uncle-in-law are rather likely to be on FB, but not Twitter. If they are on Twitter, Twitter doesn't hector you to "connect" with them with nearly the same fervor as FB.
That has somewhat profound implications. If one ditches FB, one loses access to your family to some small or large extent.
On the mild end of things you miss out on baby pictures and invitations to picnics. On the more distressing side of things, unfriending a family member or leaving FB altogether may be seen as a rejection of parts or all of the family.
Perhaps this doesn't apply to your family, but we can agree it applies to many.
The "family" aspect of FB also makes it much more of a fertile breeding ground for misinformation relative to Twitter. The boomers using FB are (on average) much less tech-savvy and don't know how to verify claims. But, as your neighbor/uncle/mom/dad/whatever, they are much harder to ignore than some Twitter rando.