In earlier days radio systems using powerlines, telephone lines or other wire media were very common. Which of these systems, which were already in the Stalinistic era are still in use. Interestingly at some powerlines the ground conductors - not the powerline - is used for wire radio transmission as this picture shows

http://m.io.ua/img_aa/medium/1941/59/19415971.jpg
Pylon with PLC-trap connected to ground conductors

Imho, 95% of wire communications in countryside was dismounted and replaced by optics or GSM/satellite communications (they cheaper and more reliable)

Nevertheless, 5% are still in use. For which purpose? Only by power companies for internal communication?

Yes, it is only for internal communications and it is deprecated. Combination of optics and lightning-protect cable are more popular.

P.S. On railroads sometimes rails and power cable are used as wired communications. The quality of such link is not very high, but this is better than nothing :)