I tried to power both Raspberry B+ and Raspberry 2 B devices via the USB ports of both a Fritz!Box 7490 and Fritz!Box 7360.
At first this works, but the Raspberry B+ devices over time would become unstable: not being able to ping and/or boot.
So below are some links on power requirements and powering Raspberry Pi A, B, A+, B+, 2B and zero.
Fazit/TL;DR: use an external power supply when available.