When all publication and data sources were print, tracking usage of individual articles would have presented insurmountable obstacles. The convention of tracking impact by journal usage emerged from those constraints. Today however, when all data and publication is electronic, it is possible to gather far more granular evidence of impact for individual articles, as opposed to journals alone. An article's impact is measurable, independently of the journal in which it appeared.
The purpose of article level metrics is to incorporate many data sources in addition to journal citations. This generates a universe of datapoints which can be recombined in various ways to focus the picture of an article's reception and positioning in scholarly research. The Public Library of Science was the originator of article level metrics.
Altmetrics is a commercial platform which provides users with tools to research the impact of their articles.
Dimensions includes both traditional and altmetric sources. It also provides links to the Altmetric.com data for individual articles.