'Happy Birthday my good looking son in law Jack,' wrote Fergie.
'We want to see another Royal Baby Announcement!' replied an enthusiastic fan.
The couple met in 2010 and in January 2018 announced their engagement followed by their wedding in October.
"The lake was so beautiful, it was a special light and I even said 'this is an incredible moment' and then he popped the question, which was really surprising even though we had been together seven years," Eugenie revealed.
"It was the perfect moment because we knew it was going to end up this way." She added: "I was over the moon, crying."