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.
Permission not granted
Critics have noted that these apps are all available for download from Apple's iTunes store, even though the practice of copying address book contacts without prior consent appears to violate its user guidelines.
The Apple guidelines say: "Apps that read or write data outside its designated container area will be rejected."
They add: "Apps cannot transmit data about a user without obtaining the user's prior permission."
Social networks have said that data taken from smartphones - which includes names, phone numbers and email addresses - is used only to identify friends who used the same service, and notify the user.
But sometimes the data appears to be taken without first informing the user, or indicating how long the information will be saved for.
"We want to be clear and transparent in our communications with users. Along those lines, in our next app updates, which are coming soon, we are updating the language associated with Find Friends - to be more explicit," Twitter spokeswoman Carolyn Penner said.
Currently, Twitter tells users that it "may customize your account with information such as a cellphone number for the delivery of SMS messages or your address book so that we can help you find Twitter users you know".
Twitter informs iPhone users that it will "scan your contacts for people you already know on Twitter".
However, the Los Angeles Times reported that the app in fact uploads every address book contact and stores it for 18 months - something not made clear by the app.