Most meteor apps are stateless on the server so you can safely scale by adding more servers (and more server to Mongo if that's the bottleneck). This can be easily done with Cluster package (https://github.com/meteorhacks/cluster). It's so easy you don't even need to configure anything.

As the sibling comment said, you do the same way you scale anything else: add more servers that replicate the same data. As it becomes to large, shard your publications depending on type. Meteor's livequery works better serving a lot of similar subscriptions, so at some point you can have multiple server types each taking care of one scope.