I think the fact that you are at a bar, and not hunting is what makes people more open. When you are networking in an event (where everyone is), you have your guards up.

You basically think everyone is a shark trying to get to you.

When you are relaxed at a bar (alcohol helps), you think people are chilling out; and you tend to believe what the other guy is saying.

Here is another one: Airplanes. I have made some very good friends just by sitting next to someone. (and some very interesting stories).

Another one which is a win-win: High caliber girls which introduces you to their circles. Usually it's a set of another "good" girls that have boyfriends who are rather established.

Another one is being married and meeting other married people through kids activity. I'm speculating here I have never been through that.