The top three Facebook, Twitter, Google plus are great places to start. Each platform has it's own culture and expectations and will require separate approaches. For example Facebook you'll truly want to be social and focus on being interesting, helpful, and conversational. The actual promoting is secondary and works best when minimal. Even on your Facebook Page. Twitter you'd want to focus on being informative and entertaining with a few lines of text. Twitter is also a great news source for people and current events update leader. Google Plus supports images and video well and can serve as a hook or bait to get people interested. It's also easier to add people and organize them into circles.
Wordpress and Blogger are blog platforms but definitely have a legitimate network of users who will visit and interact with you through comments.
I hope this helps in some way.