Here are the final numbers
GNU Social/Mastodon: 49
What's interesting is that on Diaspora, pump, and GNU Social, I all got actual comments about how the study was being conducted. I got literally zero comments on twitter or G+. There's all sorts of bias in this, but I have more "friends" on facebook by an order of magnitude and significantly more on twitter than the free platforms. So, it seems like people join the proprietary networks and just don't use them very much.