I'm not a social chair but I'll answer the questions anyways.
The thing that makes a mixer/social good is having a good amount of people show up. The social isn't just about bonding with the fraternity or sorority but being able to bond with your own organizations as well.
Attendance makes the social successful. In order to have good attendance, it's important to plan ahead. Give a calendar out at the beginning of the year to your brothers/sisters that has all the social dates on there. If people know about the social ahead of time and can plan around it, then they will show up. Announcing at the Sunday night meetings that there is a social on Tuesday wil not create a good turn out.
My favorite mixer was one that we did with SAE during my freshman year. It was a marriage/divorce social. A great way to meet people. The social doesn't have to be extravagant. The marriage/divorce social was a simple idea, hardly costed any money, and is considered one of the best socials we have ever had. Two years later, my sorority still talks about how much fun they had on it.
Another good social was ATO's "School Days". Each room in the house was set up like a classroom with a theme for each room such as History or Math and there was a drink that accompanied the theme. In the History room, Kamikaze's were given out. Once again, a simple and inexpensive idea that turned out to be a lot of fun.