The MtGox leaks showed about 60 thousand accounts. Some of those were probably inactive or duplicates, so the real number of users is likely to be less. Now, Gox is the largest exchange outhere, so even assuming there's a percentage of users that use only other exchanges and people that use no exchanges at all, even the most generous estimates take you around the 100 thousand users mark. So I would think that's the estimate of people using Bitcoin. Some would say it's too small for it to take off. I like to call it "room to grow"