Here is a cautionary tale about verifying sources you find on social media: Dan Nainan, The Media’s Favorite ‘Millennial,’ Is 55 Years Old; To verify user pages, look for their other social pages, look at the dates the pages were established (if they just joined Instagram yesterday), check for family members, friends, geographic location evidence, check for birthdate in birthday mentions, employment, and review the content of their posts for clues. Here's a great case study using social media to investigate a person: Search for Simone: A social media investigation, by Red Goat Cyber Security. via plessas.net.
This is a clip by OSINT Curious showing how to use tools like namecheckup.com or namechk.com to help find someone's other social media accounts when you have one of their usernames or email handles. You can watch the entire video here.
LinkedIn Advanced Search (click on "All Filters") searches for users by name, location, title, current or past employment (it's often good to talk to past employees when current ones can't or won't talk to you), and keywords.
SignalHire Chrome extension works with LinkedIn and reveals emails and phones in a screen, over LinkedIn profiles. After you add chrome ext for SignalHire, sign up at the SignalHire website to activate an account for 5 free searches per month.
Lusha has a LinkedIn Chrome extension that reveals emails and phones in a screen, over LinkedIn profiles. After you add chrome ext for Lusha, sign up at Lusha.co to activate an account for 5 free searches per month.
Improver chrome extension gets you another 10 free searches per month.