Couldn't they have picked a better platform than Facebook to try this out on, given all of the research into the effect of social media on one's mental state?
Regardless of those, we definitely plan to develop non-FB options in the near future. There's a mailing list here if you'd like to find out when those are offered: https://www.woebot.io/#besides-facebook
On a personal note, I oft recommend the "Kill News Feed" Chrome extension for Facebook users. (Woebot doesn't recommend it as he doesn't know about it yet, he lives in a happy world free of news feeds :).
Given that research, wouldn't that make Facebook be the best platform for an anti-depression chat-bot? That's where the users will be.