I personally think it's social media. I have a different perspective - I am a piano player, and all the way back to the 90's I remember it was 50/50 girls / boys playing piano.
If you guys don't remember that about guitar then yeah, it's changed, and it's not just social media "exposing" the already-existing fact. The role social media has played, in my opinion is the role of "inspiring" other girls to pick up the guitar.