This Business Insider article ‘Facebook is quietly buying information from data brokers about its users’ offline lives‘ is a huge eye opener on the business model of social network companies.
In order to enhance significantly the commercial value of your profile (which they resell directly or indirectly to advertisers), they complete it by buying additional data not directly from your usage of the platform!
And the interesting aspect of it is that it appears to be practically impossible to opt out of all these data gathering sources.
Anyway, while I was quite sure that all those social networks would have had enough of all the data handed to them by us using their service, they seem to need more still. Scary, isn’t it? Maybe an area for regulation soon!