Founded in 1980, the Anxiety and Depression Association of America is a nonprofit organization in Silver Spring, Maryland. They function as an educational and research organization that advocates for addressing anxiety, depression, and other related disorders. They also maintain online peer communities for anxiety and depression through a partnership with HealthUnlocked, a social media network centered on health operated by PPD Inc, which is a subsidiary of Thermo Fisher Scientific: https://healthunlocked.com/anxiety-depression-support
As of the writing of this sentence, there are 106,455 members and 97,968 posts, with more added every few hours.
Note that posts are still indexed by search engines, so consider how much you value anonymity before participating. Medical information isn’t just more personally sensitive than your Instagram pics, it can also function as an identifier like a fingerprint if you describe the same set of symptoms in the same way. They also collect some data for research and collaboration with private partners, including pharmaceutical organizations.
I haven’t found any controversies online about HealthUnlocked or the ADAA peer communities specifically, but it would be prudent to apply the same precautions you would if you were trying to be anonymous on any other platform.
Remember that unverifiable anecdotes shouldn’t be taken as certain facts, all online communities can become echo chambers that either confirm your biases or judge without knowing the details of your life, every single database can be hacked or leaked, and no online site’s service should be interpreted as medical advice that substitutes for the opinions of healthcare professionals.