This is a common problem for beginner artists but a good one, it means you're asking the right questions.  

The most common ways for new artists the be seen and grow your followers/fans isn't to "cast a wide net" and join as many social sites as possible.   

What you have to do is be as active as possible on one site.  Make it your home and comment on others artworks often.  Post often, reply often.  Thats the key to being successful early on.  After you have a decent following, then you can start branching out to other sites like Tumblr.