I tweeted one major supermarket about some vegetables that I'd got from them, they were in date but off, some very slimy mushrooms. It was bit of a tongue in cheek post, but perfectly polite, I wasn't being funny with them.
They asked me to message them the details via a private message, with a copy of my receipt to prove I'd bought them and when. I think I attached a photo too.
They sent me a gift card with a refund loaded on it and a little extra for the inconvenience. I was very impressed! I didn't even mention a refund initially and wasn't expecting one.