Osbourne -- who was a judge on "The X Factor," though not when One Direction formed on the show -- posted to Instagram Thursday ... saying her heart goes out to Liam, and claiming the whole industry let him down.

She says LP was just a kid when he came up in the music world ... and, she doesn't think there was enough protection for him -- asking "Who was in your corner?"

Robbie Williams -- a rock star in his own right -- posted about how he met Liam and the whole 1D team back in the day ... calling all the young men "cheeky and lovely."

Williams says he particularly connected with Liam in the years after as the young singer dealt with mental health, drug and alcohol issues similar to his own. Robbie says he tried to help Liam through his trials since he could empathize with him ... adding he reached out a lot over the years.

The Take That member reminded fans no one really knows what's going on in anyone else's lives ... so, sometimes it's hard to say what pain others may be living with.

He talked about his own relapse when he was Liam's age ... saying he remembers actor Heath Ledger passing and simply thinking he would go the same way next.

Ed Sheeran -- who was close with Liam and his One Direction bandmates having written songs for the band -- issued his own statement Friday. In his tribute, Ed said he was "at a loss for words," sending his love to LP's family and loved ones.

He added ... "Every memory I have with him is a great one, just such a heartbreaking situation."

As you know ... Liam fell to his death Wednesday from a third-floor balcony at his hotel in Argentina. At least one Buenos Aires law enforcement official has said Liam jumped.

A hotel employee called authorities before Liam's death ... saying a guest was "overwhelmed" by substances and the staff hadn't been able to access his room for several days.

Tons of celebs have released heartfelt tributes to Liam ... including all of his 1D bandmates. He was 31.

RIP