You're assuming it will be anywhere close to accurate. Knowing Facebook's previous attempts at AI, I wouldn't expect much. I wonder what the false positive rate will be.

"Dave, you should stop thinking about killing yourself!"

- "But...I wasn't thinking about killing myself?!"

"Come on, Dave, deep down you know you want to. You can't fool me! I know everything about you."

"- Well, now that I think about it, watching Facebook all day does make me feel quite depressed..."