Correct me if I'm wrong but I think majority of Ukraine and Belarus people speak russian only as a second language.

So we then we should count ukrainians and belorussians separately.

Russian is also an official language in Belarus, so you should count them in. Ukraine is a different story.

You're wrong:

If we're counting this by language, we should figure out how much Ukrainians (or Belorusians) use internet in russian language, primary, and how much don't; and then we should sum up those numbers. Everything else would be a speculation.