If I were the government I would definitely operate a site such as riseup (and monitor the communication of the individuals using it). And even if it's not operated by the government, as the servers are based in the US, they are affected by things such as National Security Letters (http://en.wikipedia.org/wiki/National_Security_Letter).

As an activist I would use something such as TorChat (http://code.google.com/p/torchat/), RetroShare (http://retroshare.sourceforge.net/), or Syndie (http://syndie.i2p2.de/) for communication instead.