5-6 years ago I was a part of a 30-ish person Skype group that organized a weekly "30 Minute Music Challenge". Every Sunday, a loose theme would be announced ("loss", "slow music", etc), and then you had 30 minutes to write, perform, and mix/render a song and drop it into the Skype chat. After 30 minutes, we'd all join a voice call where each song would be played in succession and anyone could give short impromptu feedback (usually just in the form of encouragement/amazement at whoever managed to pull off something decent in such a short timeframe).

It was pretty great because it was small enough to where we could have special themes like "imitate the style of another artist in the group" and we actually knew each other well enough to where you could do that, and then when listening, you could tell pretty easily "ah, this person is imitating Wallacoloo's style". And even though what you make over the course of 30 minutes is undeniably crap, sometimes the core of it would make for material that you could expand into an actual song over the next week.

The bronies also had a similar take on this type of thing titled Toast Beard. Whenever a new episode of My Little Pony aired, you'd have a week to create an "episode response" - e.g. remixing any musical acts from that show, vocal splicing the characters, or original works that were related in some other way.

Both of these are, to my knowledge, defunct (although, it's possible they just evolved into different things; I didn't keep up). But they were really fun because they were small groups both with a lot of personality, not serious in the least, and they encouraged a good deal of experimentation. It certainly was something you did for the experience and where the outcome really didn't matter a whole lot.

This is a great concept for musicians, however I’m sure it took awhile to build up the user base. Cool to see more niche music communities floating around the web. I run a similar site www.synthshare.com except for producers to get feedback on their music.