Instagram is the social media platform I enjoy the most at the moment. It isn't without it's problems, but some good friends and some of the world's best photographers share images there. I post some of my most recent images there regularly, daily if I can (www.instagram.com/alastairarthur).
Aside from the sheer scale of the potential audience, I like too that Instagram isn't just photographers unlike say Flickr or 500px. From a street photography perspective in particular, here are a few accounts I recommend checking out:
I have to mention Bruce Gilden too. I'm personally not a fan of his harsh, confrontational style, but he is one of the most influential and well-known, if controversial, street photographers shooting today.
Of course this isn't a definitive list. There are also many 'collectives' that share great street photography, and other accounts such as Magnum Photos that often include street images as part of their amazing catalogue. Some of these photographers share mostly phone shots, some only from cameras, but you can see distinct styles and creativity. I tend not to recommend anyone that doesn't follow anyone else (it's meant to be a social network, not a gallery), or anyone that follows thousands of other accounts (they'll rarely see your images, if ever, among everything else on their feed).
It might be getting increasingly difficult to gain followers (without 'buying' them), but enjoy it for the variety and social interaction and Instagram is still a great source of ideas and connections.