As an alternative to traditional citation counts, altmetrics attempt to capture the impact of scholarly works in science blogs, mainstream and social media sites.
You may have noticed when searching in Scopus a new Altmetric widget displaying the number of such mentions about an article. Click through to read the original blog and facebook posts, tweets and reviews. For example, this screen shot shows a Scopus record for a recent New England Journal of Medicine article:
For more on the topic—including its opportunities and controversies—see the Association of College & Research Libraries (ACRL) summary: Keeping Up With… Altmetrics.