Online Writing Sites?

There's Wattpad, like Anon said. Wattpad is the site you're looking for plus social media elements; it's newer and more interactive than my next recommendation, which is fanfiction.net/fictionpress. Fanfiction.net (and, I suppose, fictionpress) is an internet staple that fan authors have flocked to for over a decade. It's tried and true and doesn't make any attempt to be a social experience. You have archiveofourown.org, which I honestly don't know much about. It appears to be a more organized version of fanfiction.net. For all of these sites, you'd best encourage reviews/reader interaction in a short note at the end of whatever you've published. People can be annoyingly unhelpful. Last of all, you have critiquecircle.com, a site whose culture is geared towards nitpicking/tough-love-style improvement in the same way Wikipedia's community is geared towards making the best articles. I hope this helps you get started!

Wattpad. I don't use Wattpad solely because I want people to read the final product, but I know plenty of writers who submit their work on Wattpad as they go along.

Its called a blog.