[Kuiper]

There are a number of sites where people post serial fiction as it's written, chapter by chapter, and many popular web serials began with opening chapters of around 500-2000 words (2-8 pages). These sites usually offer writers validation in the form of likes or star ratings, a view counter, and comments from readers (who will often respond with short affirming messages like "looking forward to chapter 2" or speculation about where the story might be headed).

Royal Road is one of the newer and faster-growing sites, notable for being the biggest home of free LitRPG stories. Archive of Our Own (AO3) is a site mainly for fan works, though there are a number of people posting original fiction there. AO3 is currently a closed site with a queue to get in (most stories can be read by anyone, but an account is required to post), there's usually a waiting period of around 10 days after you submit your email address to the waiting list.

There are also some forum-style sites like Sufficient Velocity (and its progenitor Spacebattles), where the forum software allows authors to "threadmark" certain posts in a thread and label them as chapters to separate them from the posts that are discussions/comments. Readers can choose between "Reader mode" (which just displays the posts that have been labeled as chapters by the original poster) or a regular forum mode (where they can see all of the comments from other posters between chapters). Although this forum-based approach might make it feel like it's less optimized for sharing stories (as opposed to sites that are tailored specifically to that purpose), I've found that the forum setting makes people much more likely to comment on stories. While these forum-based websites might not have the biggest audiences, they certainly seem to have the most engaged audiences, which I think makes them more valuable if you're looking to validate a story idea.