finding the impact of an article can be tricky as there are a ton of things to look at and all of them give partial pictures as to the total impact. First would be page views of the article itself, which would be the biggest factor in determining impact. Other factors would be the articles ranking on Google when searching for related keywords, how many places the article has been linked to and on which sites. If it was linked on Facebook/Twitter how many friends/followers of the linker is important, if it was put on Reddit or similar sites how many up-votes it got/how long it stayed on the front page are important. Finally, determining if any articles have been written in response/quote the article is another big factor.
actually determining the effect any of these factors have is extremely challenging, and if you manage to do it the you are looking at a huge payday. Companies highly value this information so finding ways to quantify everything is valuable information and not likely to be freely available.