facebook groups, and forums. twitter
linkedin.com is a great platform to network with professionals in your field of interest Highly recommended
also look at betternetworker.com- but be aware that the admin of this site is a marketing giant, and you might have to pay for membership among other things.
Warrior forum is a great place also. especially if you're mostly networking online Highly recommended, but I think there might be a small one time fee for lifetime membership, used to be free, but worth it either way.