Here's a quick cheatsheet showing how to tag users on Twitter, Facebook, Instagram, Linkedin, and Google Plus. Please tag only when it makes sense: Don't spam! Examples of cases when it makes sense: When you quoted this person, when you referred to that person in an article, when you met the person in the conference and have uploaded a photo of you two, etc.
Thoughtful social media tagging is crucial for relationship building! You want those influencers to see your visual quote: By tagging, you send them a notification (in many cases it will come through the social media dashboard as well as an email!)