@Yukari Big ones: mastodon.social and mastodon.online (both currently closed though). @stux 's https://mstdn.social/about , https://masto.ai/about and https://mastodon.coffee/about . If they are into tech I really like @nova 's https://hachyderm.io/about . https://toot.cafe currently closed. https://joinmastodon.org/servers and https://instances.social are good places to start, but if they are on twitter, scan their followers using https://pruvisto.org/debirdify/ to find their friends and their servers.