Earlier quoted context omitted.
Here's the paper: http://dash.harvard.edu/bitstream/handle/1/8852185/Kohen.pdf... The first sentence is: "Tampons are used by up to 70 percent of menstruating women in the United States today, and the average woman may use as many as 16,800 tampons in her lifetime."
Using tampons doesn't rule out using pads, too. If I use a tampon one day, I'm one of that 70% of menstruating women that uses tampons. If I use a pad when I swap out that tampon, I'm now also one of the x% of menstruating women that uses pads.
And here is another statistic: https://www.ncbi.nlm.nih.gov/pubmed/22980411
They're both not clear though I do think it's fair to say tampons are preferred. Just google for "tampons vs. pads".