There are plenty of social networks. To start, there are over 1 million Ning social networks, I've been told and plenty of niche social networking sites. Depending how you define "hottest" is always a question of what you like. The amount of people on the network does not mean it's the best, the sites that are most well marketed or publicized are not always the best for social networking. If you can connect with one person you wouldn't have on a social networking tool that had 5 people on it, then would that be hotter for you, than the one that had millions? I do plenty of research on the topic of social networks and networking sites and tools.

I'm working on launching one for college students and alumni called FreezeCrowd, and I'm pretty sure there are a few that are already out there, such as location based ones Foursquare and Gowalla. Google has something called Google Buzz which has gotten some buzz lately. Pip.io and Gravity are interesting sites as well. Interested in virtual groups, then there is SnapGroups. If you're an entrepreneur, you might want to check out Sprouter.com or LavaPeople.com or Efactor.com. Peoplesound looks kind of interesting from the little I've played around with it. If you like to let people know what you're shopping for, you might want to try out Blippy, WhosRich.me, Justbought.it or IhaveGot.com. If you like to take pictures of yourself and show them off to others, then you might find yourself on DailyBooth.com. If you like social video chat with friends, then you might find yourself on 6Rounds and if you want to chat with complete strangers, then you'll be on ChatRoulette, ShufflePeople or StirChat. If you're interested in collecting stuff on the Web and sharing with sources or followers, then there is Amplify.com or ZooTool or CorkShare. If you're interested in soccer, you might want to try Goalazo.com or if you have a mental illness and want to connect with others, then there is MentalHealthSocial.com or Failin.gs is where people can analyze what's wrong with you. If you like to talk about how you're feeling, then check out Moodify.me. If you're interested in linking your physical objects to the Web, then there is TalesOfThings.com. If you like to go out and meet new people in your neighborhood, maybe something like CribSocial or MeetTheNeighbors would interest you. If you like to share your business likes, you might find yourself on AlikeList.com. If you're an academic researcher, then you might find yourself on Academia.edu. You like to vote on polls, maybe you'd like SodaHead, or if you're into sports trivia, then there are sites like FanIQ. If you travel, maybe something like TripWolf would interest you. If you speak Swahili, then you might find yourself on Mawindo. I've heard something about one called Diaspora in development as well, yet it's taking a different angle on things, something like an open source network.