Most GMs start playing between ages 5 and 12, and they use a variety of tools. I'm sure almost any GM of the past 5 years has used the site at one point or another but it wasn't the lone thing they needed to achieve the grandmaster title.
Has Anyone started as a Beginner on chess.com and went on to become a GM ?

No but I'll tell you one thing, Hikaru Nakamura stays sharp by using the tactics trainer on Chess.com.

Are there REALLY players that learn chess entirely on chess.com
or isn't it more common for someone to learn the game elsewhere, get addicted, play a lot of blitz games (online and OTB)
they study and become a serious OTB competitor.
if that kind of person is what the OP means- surely you can add Naka to the list and Many others. they all played (and play) on chess.com.
what Doesn't usually happen though is for those kind of guys to waste a lot of time on the forums.
so its not like they are going to say "yes, I learned chess at chess.com!".

As I have said on another forum I have learned to play Chess from a book "Chess for Beginners" I think was the name of it. (back in the late 60's) long before Computers are what they are today. I've never played OTB chess. I don't ever expect to be a GM. This doesn't stop me having fun on chess.com. I feel it is an excellent tool for any player. OTB will always be the best for Tournament.

That is impressive, I wish if u write about the way you took it to get that level in just a short time

No but I'll tell you one thing, Hikaru Nakamura stays sharp by using the tactics trainer on Chess.com.
BAHAHAHAHAHAHA!!!
WOULD SURELY LIKE TO HEAR FROM THAT PERSON ABOUT HIS JOURNEY.