From a PLOS Blog Post:
PLOS was an early adopter and advocate of these metrics [altmetrics], with its internally-developed Article Level Metrics (ALM) platform among the earliest implementations to launch in 2009. By displaying data from multiple sources, PLOS ALM showed how an article was being read, downloaded, cited, discussed and shared, accessible from the article itself by anyone.
In fact, the altmetrics movement has been so successful that it has spawned a market of providers who specialize in collecting and curating metrics and metadata about how research outputs are used and discussed.
One of these services, in particular, has far outpaced the reach and capabilities of ALM, and PLOS is now excited to pass the baton of our altmetrics operations to the experts at Altmetric.