This is a good thread.
I have an Instagram account. It's my only social media account. I used to upload onto Flickr, but I find Instagram so much easier to use. Do I really care how many "Likes" I get. Not really. For me, I use it as a tool to help me critique my own shots - to go through a days worth of shots on a solo photo walk, for example, and post what I think are my ten best. If no one cares, neither do I - I just like the fact it pushes me to critique myself. When I get actual feedback, it's a plus. When a "real" photographer begins to follow me, that's also a plus.
I don't have the money for a Leica. I use free, or cheap old digicams with CCD sensors, and old film cameras. I like the challenge of using these old dogs to practice composition, framing, et al. Sometimes I surprise myself. Sometimes I find the cameras I use are not enough for what I want to achieve and accomplish, on a photo walk (a Nikon D40/50/70 is on my radar, these days!) But when I get a good shot from an old, cheap 35mm point and shoot, or an old CCD digicam I got for free online, it's a personal victory. Those what I post and "share", via Instagram.
It's personal for me. I like to ramble. Be outdoors. I like streetphotography, I like the visual and audible noise of the street. I love to hike, and bring a camera there, too, but it sometimes gets in the way. It never does, on the streets.
Do it because you like doing it. As one poster said above, Vivian Maier did it for herself. She never would have thought so many of us would see - and love - her work!