The actual viewership was 18.2 million as the official number per article below.
http://www.hollywoodreporter.com/live-feed/world-cup-ratings-usa-portugal-714017
Again, you can't compare it to the WS because nobody cares about the WS unless you are a fan of one or both clubs playing. The WC the entire country gets involved.
My wife is a perfect example, could care less about any sport that does not involve the NFL or our kids. She follows world cup soccer because the roots for the team and her country. After the cup is over you can't pay her to watch a sport unless it's the NFL which she watches every game on TV that she can.
Until of course, 4 years from now the WC is on. This is the average US soccer fan.
In my previous post, the numbers I looked up states that there were 13 million soccer players in the US of all ages and levels. Why are there not at least 13 million people watching each MLS game when they are on?? Hockey ratings are better and there are only 600K registered hockey players and coaches. Why?