As far as I remember, “fediverse” was actively used in identica groups. But, unfortunately, after a thorough search of web.archive.org for instances around that time, none were archived. The only ones archived were the Twitter ones, users who crossposted to Twitter.
T_T
Some of those Twitter posts with “fediverse”, came from Laconica/StatusNet instances. Some of them were very familiar. It started as “identiverse” before it turned into “fediverse”. I remember there was a discussion about it. Someone raised how “identiverse” no longer fits as there are other instances besides identica already (not to mention, other software, and identica is attached to Laconica/StatusNet).
So, yeah, sadly, that's history that can no longer be “verified”. I don't even trust what I remember, LOL. It was a time when everyone was just cool about it, agrees easily, and then poof, it's done. It didn't register well in our memories.