In the U.S. The admissibility of evidence is based on relevance.[1] The modern formulation asks: does consideration of a piece of evidence increase or decrease the probability of some material fact being true?

A retweet is certainly relevant. The evidence makes it more likely that two people are friends than the baseline where there is no retweet.

[1] http://en.m.wikipedia.org/wiki/Evidence_under_Bayes_theorem