Grab twitter ids based on emails like fliptop or fullcontact

Question

How do these APIs use email to grab twitter id which is not public available?

For example,

http://www.fliptop.com/peoplegraph
http://www.fullcontact.com/developer/docs/person/

Answer 1

Kipp here, from FullContact. To get our data, we combine information from hundreds of public websites, social networks, APIs, and trusted partners. In essence, we take lots of different data points and process them algorithmically to build clean & accurate contact records.

More info at: How Does FullContact Work?