"Instagram is down! Such a shame because I'm hungry but now there's no point in eating until it is back up," MetaFilter founder Matt Haughey said.
"Instagram must have taken the ice bucket challenge," said YouTube personality Claire Marshall.
"What's happening with your food? Are your dogs and cats okay?" TV producer Elan Gale posted.
"Instagram isn't working. Thanks Obama," tweeted an account named Thanks Obama.
Ricky Gervais claimed responsibility: "I can't believe I crashed Instagram. Really sorry."
The app's makers acknowledged the problem. "Instagram is experiencing some technical issues. We're aware and working on a fix," Instagram said on Twitter.
Around 2:30 p.m., many users were saying Instagram was working for them again, and the tweet acknowledging the technical issues disappeared.
An Instagram representative did not reply to an email asking for details of the outage.