I'm not entirely sure if there was a problem with Gone Wild Stories that made /r/SexStories come out of the woodwork, but with 1/5th of the readership and nowhere near as many updates, I'd imagine that it hasn't exactly caught on by people who are disappointed with the content or moderation decisions of the team over at /r/GoneWildStories. From what I can tell, fiction is allowed here on Sexy Stories, although you have to make it clear that you're not submitting a true story – I guess that's one difference between the two subreddits.
In the last week, only 25 stories have been added to the subreddit, which is relatively small. The content is voted on quite a bit though, and good submissions are separated from those that aren't all that great. Stories with parts are quite popular here – people will submit an introduction then, over the next few weeks and months, continue adding to the tale. This goes for both non-fiction and fiction entries, with series like 'I'm too young for sex' and 'The Sitter' getting quite a few followers and upvotes since they were submitted to the community.
Reddit is a great platform, and Sexy Stories is a good subreddit, but I think it has to come second-best to the much more active Gone Wild Stories. You should probably subscribe to both if you have an account, but I'd only be dropping by here every month or so to see if there's anything new – it doesn't update all that often to justify coming any more often than that.