Online video site Hulu has been losing viewers since implementing its subscription model in June 2010. Hulu had about 27.5 million unique viewers in March 2011, according to ComScore – a 31% drop in unique viewers from March 2010.Sounds bad, right — more evidence that adding a paywall is a site killer?Except that one metric, whether it’s unique visits, pageviews, or video views, doesn’t tell the whole story. In fact, Hulu’s revenues were up 90% in Q1 2011, according to a blog post by CEO Jason Kilar.  Adding another revenue stream in the form of online subscriptions must have contributed to that growth.The site’s paywall hasn’t killed advertising revenues, either: Hulu was the top site on the Net for video ads viewed in March, also according to ComScore.I’m not privy to Hulu’s detailed financials so I don’t know the exact impact their subscription model is having on revenues, or whether the model will succeed in the long run. But I do know that it’s easy to latch onto one metric – like pageviews or site visits – and make broad conclusions about a site’s health or the viability of paywalls in general. In reality, though, sites with a hybrid subscription/advertising model are always balancing the two revenue streams, and declines in one metric may be offset by improvements in others.You can’t get a complete picture just by looking at a single number.