#18 : rise_UIED : late 2012 is when I discovered it.
Lol, Lichess was already written in Scala then. Either way, you can look at people's profiles to find out when they registered.
This account right here (Hellball) is one of the oldest from when lichess had <50 players connected at a time. It was intentionally minimalistic too, but then "features happened" in Thibault's own words. :^)