The argument is that online sharing does not invoke the same behaviours as traditional methods and consequently needs it's own set of metrics. There are different characteristics to the behaviours and these generate different data that that traditionally collated by impact factor generators, therefore creating the need for a new appriach to measuring impact > Altmetrics..
For example Elsevier clusters like kinds of activity for this type of reporting, rather then compressing all activities into one.
Image from : http://altmetrics.org/wp-content/uploads/2010/10/four-ways-to-measure-impact-copy.png
Leaders in the field includeAltmetrics
Impact story .org (free (plugs into Google Scholar My Citations)
Some of these interact with other services such as Vivo, Profiles, SciVal, Experts, and Figshare.
Altmetrics are starting to appear in subscription resources as sub products or reports.
Altmetrics are a way for crowd sourced / funded projects to measure their success in reaching their targets. They enable researchers to be able to assess whether they are reaching the right market / population with outreach and gives context to the research / paper.
Recent trendTweetable abstract submissions
Spaces to watchgrowkudos.com
Counter (use in Institutional Repositories).
Read more at : http://altmetrics.org/manifesto/