The port is not active and cannot be used for any data purposes. In fact, plugging something into can crash the STB.
what is the usb port for on the stb? which esata hard drive is compatable with fios?
It's for nothing. Its there if they ever want to use it for something.
This page will tell you about esata drives.
http://www.verizon.com/Support/Residential/tv/fiostv/other+hardware/esata/129841.htm#
You can (and I often do) use the USB to charge devices. I have never had an STB issue while connecting my phone to the STB to charge.
I do charge some devices on my STB. But found when I tried one of my Game controllers, the box crashed.