Pepsi is facing a backlash for changing the formula of Diet Pepsi.Some customers are complaining that the new Diet Pepsi, which hit stores last week, tastes "terrible" and has a strange aftertaste."Diet Pepsi is awful," one customer wrote on the company's Facebook page. "I hate the new flavor."Another wrote, "Got a bunch of the new diet Pepsi. Disgusting. Will have to return. Bring the old one back."The revamped formula replaces the artificial sweetener aspartame with a blend of sucralose ' best known by the brand name Splenda ' and acesulfame potassium.Pepsi rolled out the new formula following years of declines in soda sales. Diet Pepsi's sales fell 5.2% last year.The company hopes that getting rid of aspartame, a widely maligned sweetener, will boost sales as Americans increasingly seek out teas and energy drinks over soda.The reactions to the new formula on social media contradict what Pepsi found during internal testing.Three quarters of consumers who tested the newformulaliked the taste and responded favorably, according to Pepsi spokeswoman Elisa Baker."We're really confident that consumers will love aspartame-freeDiet Pepsi," she said, noting that the drink has only been on store shelves for a week."We're keeping a close eye on feedback, which is what prompted us to change the formula," Baker said.At an investor conference in February,Pepsi North America CEO Al Careynoted that"The number one thing we see from consumers is a complaint about aspartame." "Aspartameis just onesweetener but it's the one that seems to get most of the negatives in the press and on YouTube," he said.But now some customers are begging Pepsi to bring back the aspartame:So #dietpepsi tastes terrible now... Get that spike in sales @pepsi then ultra classic diet Pepsi super can be released.' matthew cook (@matthewcooksays) August 18, 2015They've ruined @dietpepsi! The new "aspartame free" #DietPepsi has an AWFUL aftertaste. I'm switching to @DietCoke and not looking back!' Jimmy Mack (@jcmack03) August 18, 2015Anybody else secretly thanking Diet Pepsi for removing aspartame' I can finally kick my addiction simply because it tastes so terrible.' JLaufe (@JLaufe) August 18, 2015@pepsi the new diet Pepsi has a bad after taste... Ick' Justin Silver (@JustinSilver915) August 17, 2015Holy sucralose, batman. The new @dietpepsi is terrible. Thanks for helping me kick my cola habit, @PepsiCo.' Kelly L (@SmelyPiratHookr) August 18, 2015Mission complete. I found some Diet Pepsi with the old formula! Day made. Now to get rid of the case I bought with the new formula. @pepsi' Bonnie Smith (@Vegaswench_TF) August 18, 2015I want my aspartame-laden Caffeine Free Diet @Pepsi back! The Splenda version leaves a horrible aftertaste.' Emily Post-Haste (@mlirhar) August 17, 2015@pepsi I'm Diet Pepsi drinker & I HATE the new Diet Pepsi. There is definitely a taste difference. It tastes flat!!' #IfItAintBrokeDontFixIt' Teresa Gately (@tgately) August 17, 2015@pepsi avid diet Pepsi drinker, extremely disappointed with the change in flavor' Madonna and Taylor (@Launterfied2) August 17, 2015@pepsi The new #DietPepsi is terrible. What happened' I'm addicted to the REAL Diet Pepsi. #BringAspartameBack pic.twitter.com/r1onfAq1yq' iMarty (@Krawko) August 17, 2015SEE ALSO:One alarming reason Walmart's profit is downJoin the conversation about this storyNOW WATCH: More trouble for Subway's Jared Fogle... Click here to read full news..