Scrobbling usage differs from radio usage.
In order to stream via the radio, you're effectively downloading at a constant rate of 128kbps, which is 16Kilobytes a second.
Given the metadata that goes on top of this, you'd be using about 60MB an hour.
If this poses an issue on your network speed, this suggests that your connection isn't really that fast. I'd inquire with whomever maintains your network to find out just how fast your connection is and to see how much of it you're using up by streaming.
clipped from: www.last.fm 