"Silver" fillings contain mercury; they are often referred to as "mercury amalgam" fillings. And yes, they are still in common use, though many dentists are switching to white fillings which have no mercury, and many people who have silver fillings are having them removed and replaced with white fillings.
Yes, mercury is poisonous -- even a small amount can do great harm. Yes, the use of mercury in dental work has been linked to many health problems.
Some dentists believe that because the mercury is such a small amount and it is contained within the filling, it is harmless.