I think flaking is far more common with OLD than it is with someone you met in a normal in person setting. I also feel that this is somewhat common from numbers you get at a bar, I'm not basing this just off of personal experience but from observations as well.
Women today are just flaky in general, mostly due to just how many men they have as options. You also need to realize that women, like men, may have other stuff going on in their life that have nothing to do with dating. Do you think you'd be a priority if she is in the middle of finishing up her nursing degree or insert whatever life event when she doesn't even know you? Of course not.
With that said, I've had women ask ME out and proceed to flake on me out of the blue on a scheduled date and then ghost. Women may do this due to having tons of options, wanting to lead men on or perhaps it's just the game they play to put themselves in a position of power.
I've personally found that at the first sign of a flake it's better to just delete the number. On a side note, this is just the way women operate now and chances are they aren't all that into you in the first place.
