http://isdb4l77sjqoy2qq7ipum6x3at6hyn3jmxfx4zdhc72ufbmuq4ilwkqd.onion
When users have persistent identities, even if this is just a random number, like a Session ID, there is a risk that the provider or an attacker can observe how the users are connected and how many messages they send. They could then correlate this information with the existing public social networks, and determine some real identities.