According to Wikipedia, the monthly viewership was at 4 million by the end of season 1. And that is only via conventional broadcasts and stuff Hasbro has tabs on (so no YouTube, torrents, or livestreams). I myself have only been in on this for just over three weeks, and I've talked to friends who say they have siblings/other friends who are also into the show, so it seems to have spread really quickly over the last 15 months.
Really though, the answer is: not big enough. This show deserves everyone's attention. Not saying be pushy about it, but I'd advise against hiding it as well.