Probably the same reason people were annoyed when they found out the delicious new indie beer they liked called Blue Moon was actually owned by Coors.

Or that delicious small batch whiskey maker is actually owned by Diageo, a mega corp with annual earnings of 12 billion pounds

I don't use Facebook so I can't vouch based on my own usage, but based on conversations w/ her she's been dissatisfied with Facebook for quite some time. She still checks it every day (this is the power of Facebook) but feels compelled to because it's the best way to discover events. I'd bet she spends more time on Instagram but Facebook is right up there.

Other than that she uses Group Me, which is basically a big chatroom app, it can get overwhelming though and from what I know she's muted the chats.