Not really. Do you buy drinks for your friends? Of course you do! But do you buy drinks for strangers in bars because you're interested in becoming their friend? No, that would be weird.

Why would it be weird? Because buying drinks is widely recognized as a sexual come-on. It's just not a very good one. It's not very good because it's a signal of the form "I am sufficiently desperate for company that I am willing to bribe you just to talk to me".

Anyway, it's a bad idea for all sorts of reasons, but "friendship signal" is not, I think, a good description.