R&B singer Chris Brown has taken down his Twitter account after a vulgar online exchange with comedian Jenny Johnson, The Associated Press reports. Johnson says she's now receiving death threats on Twitter from Brown's supporters. The fight started when Brown, 23, posted a picture of himself, saying he looked old. Johnson replied: "I know! Being a worthless piece of ---- can really age a person." Brown fired back a series of crude sexual jokes. Johnson posted a link to a news story about Brown's 2009 assault on singer Rihanna. Before deleting his account, Brown wrote: "Just ask Rihanna if she mad??????"