No "promise" will make me share my diary with a stranger, not even a legally enforceable one, unless I self-censor my diary. Honestly not sure why anyone would entrust their diary to some web service, unless they take a nothing-to-hide approach with their diaries.
Although, a web service to share notes with friends is probably okay.
"Diary" usually means personal and private. But it can mean a lot of things. I can imagine cases where people are okay with that. I've managed a personal "diary" that's on Github publically. It's really just a reference of tech stuff I've learned.
Although, a web service to share notes with friends is probably okay.