Facebook has become the de-facto standard in "social networking", and approximately 90% of 18-to-49-year-old people in Germany, the EU and the Americas are using it frequently. Moreover, it has more than 2.5 billion active members worldwide (as of 2020).
Through Facebook pages and applications, these users' data are potentially available to third parties: If a user installs a Facebook app, a lot of personal data and their preferences are available to that app.
The Facebook Connect interface also allows easy sign-in at other platforms with a single click, dispensing the user from re-entering all their user data. This lowers the entry barriers for third parties and improves the user experience.
Both tools can be used for customer acquisition and offer great potential for viral marketing, as each Facebook usually holds a "circle of friends" around them.
You may also find our Google API Integration of great interest.