Is your site new, less than 6 months old? Sometimes newer sites will drop in the rankings for 6 months or so until it gains some authority and credibility in Google's view.
Is the content on your site unique, or is it copied from somewhere else? Make sure the content on your site is unique.
Can you tell where the traffic was coming from before the drop? If it was coming from the search engines, do you know what search terms they were entering and if you do are you still ranking for those terms?
If most of your traffic was coming from Facebook before, perhaps your EdgeRank score in Facebook has dropped so your posts are not being seen by as many people.
Those are a few questions to ask yourself and research.