It used to mean hanging out with real people in a face to face situation.  It doesn’t necessarily mean that you had to talk to each other.  It means at the very least that you had to be together in a social setting.

I heard some guys talking about their kids playing video games online, and they were arguing whether that was socializing or not.

The theory of Copresence is this idea that you can be “there” together – where ever “there” is.  When you play online, you can experience presence “there.”  Playing video games online with people from around the world = socializing.

See you “there.”