To be honest, I don’t really know much about the people behind either of these sites. That being said, Jester is pretty active on CoSo. I originally gave CoSo a try because Twitter was beginning to go downhill and I was looking for an alternative. I tried a few, CoSo had a lot of neat features and impressed me enough that I was even willing to pay the 4.99/month for the “Pro Account”.
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As for this site (kbin.social), I was looking for a Reddit alternative. This site reminds me of what Reddit used to be and, for that reason, looked better than Reddit does these days. I known nothing of Lemmy and had never even heard of it (him?) before finding this site.
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In both cases, you just don’t see the bot-farm-generated trolling that loads up the mainstream sites. The stuff that’s just out there to get us arguing with each other. Also nice to not see endless ads. Maybe not completely free of trolls, etc., but, since it’s not as prevalent, it’s easier to filter out.