"To the funniest, most inspiring, incredible man I know . . . @WValderrama -- thank you for helping me get through my darkest moments," Lovato, 21, wrote. "You truly are a light to this world and to my life . . . Happy Birthday!!!!"
The Twitter PDA, however, only fanned the flame on earlier rumors that the pair are engaged. On Wednesday, Jan. 29, Lovato shared a pic of herself wearing bling on her ring finger, but her rep told Us Weekly Thursday that it's not an engagement ring.
The "Skyscraper" singer and That '70s Show alum first hooked up in 2010. Their relationship status has remained unclear since then, but they haven't kept their close friendship a secret.
Last August, Valderrama wished Lovato a happy 21st birthday in a heartfelt message on Twitter. "I've been so proud of the strength & commitment you've gained 4 life, keep inspiring us all! Happy birthday my @ddlovato!" the actor wrote. "With & by u always."