Back in the early 2000's I was on a site called acidplanet.com, created by Sonic Foundry to give users of their software an outlet to post their music. It wasn't limited to people creating music using loops though, anyone could post music there.
I did many collaborations through that site, probably close to 100, adding guitar to other peoples tracks situated all across the globe. And had people contribute parts for my songs, there was a percussionist in Brazil that used to do percussion tracks for me.
At that point probably the most fun I had had in music for a very long time. Of course all good things must come to and end, and that site went down the tubes when Sonic Foundry was bought out by Sony.

From this site I have collaborated with PeterF, and also ScottC, and had lots of fun doing so.

And of course for the last few years I've been working with a singer/songwriter from Germany, she heard my music over on soundcloud and asked if I would like to do a song with her, and we have been working together ever since, 30 songs so far.

Dave

Blue Attitude Website
Blue Attitude YouTube channel