SAN FRANCISCO — On Monday, a search on Instagram, the photo-sharing site owned by Facebook, produced a torrent of anti-Semitic images and videos uploaded in the wake of Saturday’s shooting at a Pittsburgh synagogue.

A search for the word “Jews” displayed 11,696 posts with the hashtag “#jewsdid911,” claiming that Jews had orchestrated the Sept. 11 terror attacks. Other hashtags on Instagram referenced Nazi ideology, including the number 88, an abbreviation used for the Nazi salute “Heil Hitler.”

The Instagram posts demonstrated a stark reality. Over the last 10 years, Silicon Valley’s social media companies have expanded their reach and influence to the furthest corners of the world. But it has become glaringly apparent that the companies never quite understood the negative consequences of that influence nor what to do about it — and that they cannot put the genie back in the bottle.

“Social media is emboldening people to cross the line and push the envelope on what they are willing to say to provoke and to incite,” said Jonathan Albright, research director at Columbia University’s Tow Center for Digital Journalism. “The problem is clearly expanding.”