Where do Companies like SignalHire Scrape their Data?

Got a popup for signalhire today, accidentally clicked and ended up at a website selling a service that gives you all info about a person - everything from phone numbers to social media accounts after you use your 5 free credits/month. Where are they scraping this data from? It's a bit creepy

Somewhere like Ambridge Canalytica?

According to US law, as long as the info is publicly posted, it is fair game. Public social media posts and professional directories and online resume dbases come to mind. The problem isn't finding info but finding fresh information