The problem with pure peer-to-peer communication is that it is unreliable due to IP address changes and NATs, requires you to always be online (or at least both

The problem with pure peer-to-peer communication is that it is unreliable due to IP address changes and NATs, requires you to always be online (or at least both be online simultaneously) and also exposes your social graph to your ISP and any other intermediary on the network path. Some of these can be fixed by adding servers into the mix (such as discovery/rendezvous servers, or Tor), but that doesn't solve everything.