Twitter has admitted copying entire address books from smartphones and storing the data on its servers, often without customers' knowledge.
Access to the address book is enabled when users click on the "Find Friends" feature on smartphone apps.
Two US congressmen have written to Apple asking why the firm allows the practice on its iPhone, as it contravenes app developer guidelines.
The practice came to light when an app developer in Singapore, Arun Thampi, noticed that his contacts had been copied from his iPhone address book without his consent by a social network called Path.
Dave Morin, CEO of Path, apologised and said Path would ask users to opt in to share their contact information.
However, he noted separately that it was an "industry best practice" to upload or import address book information.
iPhone apps by social sites including Facebook, FourSquare, Instagram, Foodspotting and Yelp are also reported to access the address book.
However, Facebook has told the BBC that its app will only upload address information if the user opts to sync their contacts with the website.