Neverwinter Nights on AOL predates Meridian59 when it comes to these sorts of games. Of course for the non-graphical variety there were MUDs before then. I don't really know when a game goes from "lots of players" to "massively", wikipedia says Neverwinter Nights on AOL was an "MORPG" not an "MMORPG". So it kinda depends upon how you count these things I guess.
FWIW, Raph Koster ran a MUD before he worked on Ultima Online. The original creators of Everquest played MUDs too - Sojourn in particular.
Our dividing line for massive in 1995 was 250, because most LAN play capped at 64, and MUDs achieved up to around 253 (the number of available file descriptors on a standard Linux kernel at the time).
Meridian 59 launched with 250 player capacity. UO launched with around 2800.
FWIW, Raph Koster ran a MUD before he worked on Ultima Online. The original creators of Everquest played MUDs too - Sojourn in particular.