Most normal servers do not have anything defined as a backbone. They are just normal computers. On blade servers, I suppose what you could call the backbone is part of the cabinet and allows the blades to communicate internally and externally via switches, fiber channels, vga, etc. (depends on what it's capable of).
Switches have backbones - that's the speed of the internal data bus and defines how much data a switch can handle between ports (although hubs don't as they simply re-broadcast traffic, and therefore the highest bandwidth is, usually, that of the slowest port).