I am 28 and my sister (Megan) is 22. I know a couple Megans (Meghan, Meaghan) my age. I would say above average but nothing crazy. However, my sister had many other Megans around all the time. She was born in 1990, and I think it may have been at its peak then! I am a preschool teacher and have never taught a Megan in the past 6 years, but DH does have 2 cousins named Megan (4 years old and infant).
Edit: I just want to add that I love the name Megan! I would still use it... although I am confused how you get Megan from Mary Margaret? I know a couple Mary Margarets and they are all Mary Margaret, Mary, or MM.