In addition to public viewing and viewing parties (anytime more than a few people are watching one screen), it's pretty tough to get viewership numbers in some parts of the world. Even in the US, where we think we've gotten it down to a science, there is a lot of gray area and good deal of fighting over ratings and whether they are really reliable. Especially with the advent of DVRs and internet viewing.
Last year, the Super Bowl had just over 100 million viewers in North America (not just the US, but Canada and Mexico too), but only about 10 million outside North America. About 115 million worldwide. If we say the World Cup final had about 260 million viewers, that's dwarfing. Not to mention the 63 games before it.
Not to be a pissing match between two sports, the point being that, yes, the Super Bowl is big here, but here isn't everywhere.