The Defriended app gives you an easy way to track these defriending events since Facebook itself doesn't provide this feature - or at least that's what the app did until Facebook blocked its operation. Apparently, the social network thinks defriending should be a private matter. As of now, the app is no longer available for download in the App Store.
Techtechnology
Defriended KILLED: Facebook Blocks App To Track Defrienders
Defriended KILLED: Facebook Blocks App To Track Defrienders