The expression "Social Butterfly" is one of those rare things: A metaphor in which the true meaning is obfuscated by its use. At first glance you'd think "Well, someone who is a social smoker is someone who only smokes around other smokers, so a social butterfly must be a person who briefly turns into a butterfly when around others who have undergone a similar transmogrification", and I applaud you for your ruthless application of logic, but since this is a metaphor, we can throw that out of the window.
No, "Social Butterfly" is an analogy for someone who is gregarious, and socially-adept. And as we all know, no creature better personifies the affable nature of these people than the notoriously chatty butterfly, so it really is an ideal fit for this role. Can you imagine how silly you'd sound if you described someone as, say, an "Amiable Giraffe" (The giraffe, of course, being well-regarded of his stand-offish nature and well-guarded private life)? Such nonsense could surely not be tolerated in the English language.
I, for one, am not a social butterfly. I am, at best, an awkward moth. I am trying to popularise that as the opposite of a social butterfly.