Web developer and web/server security expert here. Most probably its a malicious code in your website which has triggered safety mechanisms of facebook and social medias.
Relevant Skills and Experience
Webmaster-developer for more than 10 years, server, security and network manager for 6 years. I run a few personal websites which provide download of files and come across such security flag problems.
Proposed Milestones
$55 USD - After completion of work
The website URL and if its flagged as unsafe/malicious only in Facebook and Social Medias or Google.