It helps to think of impact at four levels - article, journal, author, and law school.
Impact measures citations:
Metrics look at the ratio of citations over time. The field of law generally measures impact over a rolling five year period.
Author impact -
HeinOnline Author Impact Profile (issue: multiple names; proposed for USNWR faculty rankings)
Google Scholar Citations metrics (Author Profiles)
ImpactStory Profile Impact Story is a non-profit that creates software to promote open access scholarship. Their profiles software tracks a researcher's mentions in the news and in social media. Create a profile using your Twitter handle.
Altmetric includes "evidence of societal engagement, influence and broader impacts... including any recent publications that have not yet had a chance to accrue traditional citations", e.g., twitter, Facebook, reddit, blogs, etc., plus news media, Wikipedia, websites, books, and more.
PlumX (now owned by Elsevier) metrics count five separate categories: citations, usage, captures, mentions, and social media. Also used in bepress Digital Commons.
Journal impact factor - measures the frequency with which the “average article” in a journal has been cited in a particular year or period; a ratio between citations and recent articles published. Usually the higher the better. For example, an Impact Factor of 1.0 means that, on average, the articles published in a given journal two to five years ago have been cited one time. An Impact Factor of 2.5 means that, on average, the articles published in the time period have been cited two and a half times.
Washington & Lee Law School's Law Journal Submissions & Ranking
Allen Rostron & Nancy Levit, Information for Submitting Articles to Law Reviews & Journals
Bryce C. Newell Meta-Ranking of Flagship US Law Reviews 2018