"Conversation-friendliness: Your main tool of contact and communication is your voice. So ask yourself whether a particular venue is suitable for carrying on a conversation. Quieter places are generally better. Loud clubs and bars generally are not, but sometimes their quieter patios or balconies can work. Bookstores and coffee shops are much better.
Community: There is pre-existing rapport when some common thread of interest connects a group together. The more specific the thread and the greater effort people have made to come to the venue, the stronger the rapport, and the easier time you will have making contact. Talking to people watching a game at a bar is likely to be a fleeting interaction, whereas the people you meet at a conference in Buenos Aires on emperor penguins may become lifelong friends.
Continuity: Stationary people are easier to meet than those in motion. The longer people tend to stay at a given venue, the easier it is to meet them. Continuity can also develop over longer periods of time. There is built-in familiarity with someone whom you see three times a week at the gym which makes it easier to meet her."