Can fluoride decrease bone density?

Depends on exposure. Interesting question. It has been a long standing observation that over exposure to Fluoride can lead to decrease in bone density and increased overall fracture rate, however and interestingly enough, limited exposure actually increases bone density (mainly in trabecular bone such as spine). Even so, the increased density does not correspond to increased bone strength.
No. Decrease in bone density is not a proven side effect of fluoride. Optimally fluoridated municipal water supplies are both safe and effective in the prevention of tooth decay. See my book on amazon, the Fluoride myth: debunking the controversy.