повышение

Ищу новых друзей для повышения коммуникативных навыков.

Does для повышения mean for increasing or for improvement?

Повысить literally means put higher
So повышение is increasing/rising/growing/whatever of the kind.

But if there is a set phrase in English for "improving communication skills" rather than increasing then improvement is also okay because we should not translate words, we should translate concepts in a way that is as close as possible to natural for the reader and their language habits.