The creative SA subforum does have a flash fiction critique thread where users can post their pieces and get critiques for them. There are a bunch of regulars that are very helpful. I'd be willing to start a thread like that here if people think it would be useful!
For anyone who wants to sharpen their writing skills, they also have a weekly fiction contest called the Thunderdome that I'd also recommend. It's a great way to gauge your skill level and challenge yourself to write well with limited words, which is always a benefit.