They watched as pink-haired pop wannabe Amelia Lily was given the axe, and Little Mix and Marcus Collins were told they were going through to the final.
Spice Girl Emma Bunton wrote: "Little Mix all the way!!!! Girl power!"
This Morning presenter Phillip Schofield commented: "Well done Amelia, very gracious and nicely done. Not the last we'll see of you I think :)".
Scouser Coleen Rooney gave her support to local lad Marcus Collins and, after his performance of Billy Joel's She's Always A Woman with Gary Barlow, added: "Love it!! #alwaysawomantome".
Gemma Collins told her fans that Little Mix's duet with mentor Tulisa Contostavlos made her Saturday night.
They sang a mash-up of Empire State Of Mind and If I Ain't Got You by Alicia Keys.
And she wasn't the only fan of the hook-up; stunning glamour girl Kelly Brook said: "Tulisa and Little Mix smashed it".
Show supremo Simon Cowell posted online: "Thought @TheXFactor UK looked amazing. I think a group may win for the first time ever ... and I think @officialtulisa should be a permanent 5th member!"
Radio One DJ Fearne Cotton, tuning in with her best pal Holly Willoughby, said: "I'm watching X Factor for the first time this series. In Belfast with X Factor's biggest fan Holly!"
And Holly added: "Little Mix... T-une!!!!!!! X".
Pop star Diana Vickers, who rose to fame after she appeared on X Factor in 2008, simply wrote: "#littlemixtowin".