Anonymity is only as stong as the group you are hiding in
This article describes a clever attack against Secret, the “anonymous” secret sharing app.
Their technique allows the attacker to isolate just a single target, so any posts seen are known to be from them. The company is working on detecting and preventing this attack, but it is a hard problem.
In general, any anonymity system needs to blend the activity of a number of users so that any observed activity could have originated from any of them. For effective anonymity the number needs to be large. Just pulling from the friends in my address book who also use Secret is way too small a group.