The pop star condemned the app over a third-party ad that appeared to make light of domestic violence.

Shares of Snap are down for a second straight day after the social media giant received pointed criticism from Rihanna over a third-party ad that appeared to make light of domestic violence.

On Thursday, Snap shares plunged nearly 4 percent, wiping out roughly $800 million from its market value, according to The Wall Street Journal and CNN, and during Friday trading the stock has slid another 1-2 percent.

Snapchat came under fire earlier this week after users noticed an ad for the mobile game "Would You Rather?" which offered up two options -- "Slap Rihanna" or "Punch Chris Brown" -- that reference Brown's 2009 assault of his then-girlfriend prior to the Grammy Awards. Later that year, Brown pleaded guilty to felony assault and received five years probation and six months of community service.

On Monday, the app maker issued an apology for the ad, which was made by a third party but had to be approved in order to get posted, and said it removed it for violating the company's advertising guidelines.

But then on Thursday, Rihanna responded to the controversy with a scathing Instagram post, saying the ad shamed domestic violence victims. "This isn't about my personal feelings, cause I don't have much of them...but all the women, children, and men that have been victims of DV in the past and especially the ones who haven't made it out yet...You let us down!" she wrote. "Shame on you. Throw the whole app-oligy away."

With the issue revived in the worst possible way, Snap issued an even stronger apology and vowed to prevent it from happening again. "This advertisement is disgusting and never should have appeared on our service," the company said. "We are so sorry we made the terrible mistake of allowing it through our review process. We are investigating how that happened so that we can make sure it never happens again."

The Rihanna controversy comes less than a month after Kylie Jenner's criticism of Snapchat's redesign sent its share price down 6 percent, wiping away $1.3 billion of the company's value.