This depends on so many things.
First, what's the real reason for this comparison? If the main point is to simply figure out the different sizes of the active userbase for the two sites, then it might be better to just ask about the size of the userbase.
But if you're trying to compare profitability: e.g. how many subs do you need to make $100/mo on youtube ads versus Tapastic ad revenue? Then the answer may be quite different from a simple sub-to-userbase ratio.