Although it may have been your experience that most Libertarians are former Democrats it's typically accepted that the LP is comprised of about half from both parties. There really hasn't been a concerted effort to get an exact figure. The figure has been obtained from basically asking people to raise their hands at gatherings and a visual estimate is made. Harry Browne, who spoke in front of tens of thousands of people during his campaign, said he came to the same conclusion.
My own personal experience is the opposite of yours. I would estimate that 3 out of 4 Libertarians that I have have met at various places are former Republicans, myself included. You can read about the history of my political activism on my profile page.
The "not a dime's worth of difference" argument is often advocated by people like me who sincerely believe that the difference between the two parties is so negligible as to be totally insignificant.
It would appear the majority of libertarians once frequenting this site have moved on.