I believe that it is more like a mailing list where you can select to send messages to a subset of your contacts, and to receive messages from or filter messages from whoever you like, with the messages presented in a threaded fashion. Actually it looks like Syndie does work with any anonymity network, although it was developed by the I2P people. The user interfaces for both frost and syndie are awful though, they both look like they would probably be a challenge for the average user to use.