I browsed quite a few of those, and the first ones I read were not preventions of a mass shooting. Like the pearl high school schooting. The deaths were already done, the suspect was fleeing, and a person stopped him from fleeing, not from shooting more.
Regardless, these sort of stories would be red meat for Fox News, so why have I never seen Fox covering a 'mass shooting prevented by armed civilian'? You think that Fox, of all news sources, would love to have a week long 24/7 story about something like this.
The only thing I found on fox, was an article complaining about the lack of coverage: http://www.foxnews.com/opinion/2015/05/22/where-is-media-coverage-heroes-who-stop-mass-killings.html . And your link isn't exactly a vetted news agencies, and obviously has an agenda, given that the name of the site is "GUN WATCH" and has the 2nd amendment quoted at the top.....
So either very few of those incidents prove much either way, or there is a nationwide conspiracy to hide the truth about good guys saving people from mass shootings.