Status.net/OStatus uses a similar PUSH model, basically it scales with number of followers, and yes, if you have thousands of followers the load will be significant. As a data point, I've been running a status.net instance since last December, during which time I've had 50-80* followers, have made an average of 12 posts/day, have followed 100-190* others, and my instance has transferred a grand total of 10,279MB * over that entire period.

followers and followeds gradually increased over the duration. I have this number handy since the connection is reverse-proxied via pagekite.net, which is metered. Over the last few months, I've had all static content offloaded to another server, which reduced the bandwith used, I'm not sure by how much.