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You retain your rights to any Content you submit, post or display on or through the Services.
By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods (now known or later developed).
That sounds like users give Twitter (and with the sublicense, others) the ability to use content anyway they want. That’s what Agence France Presse (AFP) tried arguing when they were sued for copyright infringement of some pictures of the 2010 Haiti earthquake they found on Twitter. The judge didn’t agree. She ruled that
AFP’s removal from Twitter and commercial licensing of the Photos-at-Issue is not akin to the rebroadcast of a Tweet.
Instagram’s FAQ explicitly address the question of advertisers using people’s images and videos:
Does Instagram let advertisers use my photos or videos?
No. You own your own photos and videos. Advertising on Instagram doesn’t change this.
It’s hard to see how Groupon could win this case. The message is clear: if you have a business, don’t use images or other content you find on social media without permission.