Hi marcellofornari --
Our REST API includes user-level metrics (e.g. total kudos and number of posts/replies by community app -- forum, knowledge base, Q&A, blog, idea, contest) and some high-level community metrics (e.g. # of posts, # of users, logged in users). It does not include trending or analysis-based data (e.g. influential users) right now, but you can grab that via a CSV download from the Customer Intelligence Center dashboard.
Below is a link to our REST API docs so you can look at those specifics. I'd look at the Community and User classes in particular.
http://lithosphere.lithium.com/t5/Developers-Knowledge-Base/Admin-Guide-REST-API-JavaDocs/ta-p/5966