I imagine a lot of it happens in private FB groups already. I'm a member of Dynamite Circle, which is a private community of location independent entrepreneurs http://www.tropicalmba.com/innercircle/

Try mixergy

I feel twitter, facebook is very cluttered if you want a specific network like entrepreneurs. AngelList is focuses on fundraising, Clarity focuses on Tips but Teamstory - http://teamstoryapp.com will focus on the stories and moments just like instagram but for entrepreneurs.

There are many communities and groups that would likely fill your need more than dedicated platforms. Also to get straight to the point you should find exactly the type of or actual people you want to network with and reach out to them. This works surprisingly well.

The YEC is a great option: http://yec.co/ I'm a member and have gotten a lot of really solid connections, intros and press out of it.

Actually, you're thinking too hard. If you venture off to micro-niche social sites, you'll probably hear lots of crickets and get lonely. Instead, work within the biggies like Twitter and LInkedIn. At Twitter, find your contacts via subject matter hashtags; on LInkedIn, join 'Groups' in your industry and begin participating. Be patient, and cultivate genuine connections.