First, I agree that we need ways to measure scientific performance, but I am convinced that a fair future for measuring such scientific impact is only possible when a non-profit institution is responsible of developing and maintaining an open access information web portal including specialized and flexible ranking tools services, based on clear rules whether to include or not articles, and how a scientific article is defined. The whole situation is certainly too subjective. A scientific paper appearing in an important newspaper could exert a relatively strong impact in society, however e.g. see DOI: 10.1371/journal.pbio.0040055 “[W]hile I'm proud of the work, it's certainly a disproportionate amount of attention given how many other interesting things there are in science.”
Sadly such ideal scenario mentioned above is contradictory to our current trend, where ISI ranking is heavily influencing more and more the decisions inside the whole scientific community worldwide. Anyhow and after all the problem is that we have this need, the one of an impact factor to compare our peers and we have no alternatives, until now.
Could be this another goal for PLoS? May be yes.