If it was anywhere near 50 million active users, Google Reader would not have been shut down. Even if it was 25 million actives it would still be kicking. Plain and simple. Subscribing to CNN seems like the behavior of beginner RSS readers who abandon the concept. Seems like a good idea, but a general, high volume feed with more noise than signal for the average person doesn't really make sense for Google Reader user. I'd be shocked if the number of those actively reading the CNN feed was anywhere above 5 million.
For comparison, CNN on Twitter only has 7.7 million followers. Do you honestly believe 3x more people are reading CNN's RSS feed than their Twitter feed?
Yep. RSS is a better way to read news. If I subscribe to an RSS feed then I get to pick through the whole thing at the point where I feel like reading it. Twitter just blasts past so fast it's gone in minutes, I miss maybe 90% of my Twitter stream (conservative estimate, it's probably more). I realise you can go and look at an individual Twitter feed but it takes several clicks to see the whole thing and it's not as fast as RSS.