PDA alert! Demi Lovato and Wilmer Valderrama have been super secretive about their rumored on-again romance, but on Tuesday, Aug. 20 -- the X Factor judge's milestone 21st birthday -- Valderrama broke his silence to indulge in a rare public display of affection via 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, 33, tweeted to his more than 394,000 followers. "With & by u always."
The "Warrior" singer and the That '70s Show alum -- whose exes include Lindsay Lohan and Mandy Moore -- first hooked up in 2010, shortly after Lovato's breakup from her Camp Rock costar Joe Jonas. They later split but were rumored to be seeing each other again in 2011.
Since then, both stars have been linked to other people -- Lovato to One Direction boy bander Niall Horan, and Valderrama to actress Minka Kelly. Earlier this year, however, they appeared to be giving the romance another go. As Us Weekly exclusively revealed in February, the former lovebirds shared a romantic Valentine's Day meal together at Takami Sushi & Robata Restaurant in downtown L.A.
"They ordered almost everything on the menu, but they didn't touch it for at least an hour," a source told Us of the date. "They stared into each other's eyes the whole time."
Lovato, for her part, has kept mum on the relationship but hinted earlier this year that she's ready for romance. "I'm in a place where I can finally trust and love and be a better person for the man I am in a relationship with, instead of being a clingy girlfriend or getting jealous," she told Cosmo on Campus in February. "It is very unhealthy when girls devote all of their time to a guy and forget their friends and family. I did that. I was always looking for distractions because I was so afraid of being alone."