I've been lucky to be able to memorize almost all my music, so keeping eye contact is not a problem for me. I know that playing on those dark stages at weddings it sometimes didn't matter because they couldn't see us anyway. But in today's stand-up market it's essential.
I usually don't try to make personal, face contact with people - that seems too invasive for me. But last month when we played on the street I went out of my way to try to make good eye contact and talk to people passing by. In doing so, I seemed to catch the eye of a woman who seemed more interested than I was comfortable with. Whoa, Nellie... no more of that kind of eye contact.