Facebook has rolled out a new feature that allows users to select a candidate to publicly endorse on their profiles. To endorse a candidate, simply go to the page of the candidate, click "endorsements" and then create a post explaining exactly why they are supporting said candidate.
Candidates will also be able to feature certain public endorsements on their pages, but neither Clinton nor Trump has done so yet.
Facebook has also done voter registration drives, which reportedly saw a significant increase in people registering to vote.