Photo-sharing social network Instagram has come under fire for new 'terms of service' which allow it to use people's own photos in adverts, without paying them.
Twitter users have expressed outrage - "Good bye instagram. Your new terms of service are totally stupid. Good luck playing with the big boys," said one.
[Related: Best budget tablets for Christmas]
"To help us deliver interesting paid or sponsored content or promotions, you agree that a business or other entity may pay us to display your username, likeness, photos (along with any associated metadata), and/or actions you take, in connection with paid or sponsored content or promotions, without any compensation to you," says the document.
Instagram is facing increased competition from the relaunched Flickr app, which adds photo-filters, and allows users to share pictures via either Twitter or Facebook.
Technology blog Techcrunch said, "This approach can win. I predict this approach will win."
Twitter also unveiled photo filters, similar to the ones offered by Instagram, in a move which was seen as retaliation for Instagram's decision to block its files being shared within Tweets.
Earlier this month, Instagram photos stopped being visible on Twitter's site, linking back to Instagram's instead.
Kevin Systrom, Instagram's CEO, said, "We believe the best experience is for us to link back to where the content lives."
Instagram, which has 100 million users, was acquired by Facebook for $1 billion last year.
Twitter's new app offers similar features to Instagram, with filters called Warm, Cool and Vintage - like Yahoo's, these are produced by Aviary.