Earlier quoted context omitted.
Let's argue Reductio ad absurdum. Why does not Google pop-up similar warnings when you click on its search results? -Because Google is dependent on the richness and abundance of third-party websites, for its search to be meaningful. What is the objective of Facebook? - To suck users into facebook.com, and sandbox them there. Similarly, the smaller objective of Facebook Social plugins is to lift the userbase from thir…
Google scans for malicious sites as well. The difference is of course, that on FB people share links whereas Google can just decide not to include a link in the results and the user will never know.
I happen to run a well known service, and we encountered the malicious links problem. It has never even crossed our mind to display those pop-ups, instead we stop malicious links from being posted after a domain is reported or detected otherwise.