Yes, the plugin can be used on a multisite installation. You can either activate it on individual blogs or do a network activation. As users are global to a multisite installation, their choice regarding Gravatar.com use will affect all sites in the network. So if a user comments on blog A and chooses to display gravatars, this decision will be followed on blog B and C too. On new installations, comment author (i.e. non-user) opt-in is recorded per site, not per network. If you first installed Avatar Privacy 0.4 or earlier, the global table wp_avatar_privacy continues to be used for all sites in the multisite network. This behavior can be changed by the network admin in the network-wide Avatar Privacy settings, or overriden via the filter hook avatar_privacy_enable_global_table.
