By "using" ..do you mean to ask, if people are "using" bitcoins now OR how many people own bitcoins?
I am still using bitcoins on a daily basis, because it is cheap now to buy it back. You do not have to stop using bitcoins, because the price dropped. <Just make sure you keep on buying back some bitcoins at the lower price>

As for the question on how many people owns bitcoins, well that is not a question that can be answered, because Bitcoin owners are pseudo anonymous and it is impossible to count people, if you do not know who they are.
