Hardly satanists. In order to be a mason you must believe in a higher being, mostly identified as the christian god. More mainstream christians will say they are not part of them and will give them labels such as satanists, heretics, unbelievers, the usual BS that christrians like to do to any one who is not of their faith.
Calling them satanists is also an incorrect term and is misused by no end of people. The name satanist merely means a person who does not follow the Judeo-christian god. Judaism, christianity and Islam are all followers of that particular deity. Religions such as Buddhism, Hinduism and Shinto are satanists. Do not confuse the word satan in satanist meaning devil worship. It has nothing to do with it. Even LeVeyan Satanism distance itself from the christian satan.
A wiki article on them for you.
http://en.wikipedia.org/wiki/Freemasonry