I personally played it when it first came out, on Halloween! I had actually found out about the whole twitter thing a minute after Toby posted the game, I believe from Askfriskandcompany's mods answering an ask about it. The experience was very immersive that way. I had absolutely no clue what was going on, lol.... I think that was the intended effect!
The day it came out. Me and two more people were basically going at it together on each of our computers and we were psyched out that Toby Fox was seemingly making a Horroresque spin-off to Undertale and making a survey on it. It's like, out of all the RPG makers out there, half and half seem to spawn off of either Earthbound or Yumme Nikki aesthetically.
Then one of us beat the Lancer fight and it kept going and they went "GUYS IT'S UNDERTALE 2"
not sure if exactly the same day, but yeah, back when it was called something like SURVEY_PROGRAM, I've seen the twitter posts and thought wait what the heck is this? and I tried it and like WOW I need to tell someone about this