The Annals of Internal Medicine (of all places) published the results of a survey of 232 people who had arthritis and were under a rheumatologists care. Of those 63% responded to the survey by saying they were using some form of "complementary care" as named by the study. Of those people 31% were using chiropractic. These number may themselves be grossly under reported as only 45% of the patients told their doctor about using the other forms of care.
These reported numbers translate to over 19% of the public who is seeing a rheumatologists is also seeing a chiropractor. And if less that half of the patients are telling their doctor about it the actual number may be twice as high.
Possibly the most impressive statistic was that 73% of those trying chiropractic found it helpful. The reasons given why people said they tried the non-medical care was to control pain, because they heard it helps, because it is safe, because it helped someone they know, and because their prescription medication wasn't working.