LightPull is the comments plugin that doesn't grab your database.
LightPull boosts traffic to and engagement with your blog by synchronising your posts and comments with LightPull.org/l. This means people can find your blog among the others on LightPull, and can add comments with a LightPull account.
Unlike other WordPress comment plugins, it doesn’t capture your database. That means you keep control of your content, you can install and uninstall the plugin without losing all your comments, and you can add other plugins that work with your WordPress comment database. For instance, Lightpull is compatible with Jetpack, so you can use both together for even more control over your comments system.