"My son's puppy went missing about a month ago and we just found him," she wrote. "This is the video of him reuniting with his baby Kase. He was found right down the street from our house.
"A nice family had taken him in and kept him warm and fed."
Open Image Modal
The puppy had been missing for one month
The video shows Williams' son walking up to the van where his puppy is being kept, with a confused look on his face.
When he sees his puppy staring back at him in the van, he looks stunned.
Open Image Modal
The boy immediately wraps his arms round his puppy and breaks down in tears, telling him how much he loves him.
The video had been shared more than 60,000 times within a week of being uploaded on 8 January, with nearly 8,000 comments.
It prompted other parents to share photos of their children and pets' "unbreakable bond", and moved many Facebook users to tears.
"This is beautiful thank you so much for sharing, I'm in tears," wrote one person in the comments.
Another person added: "Wow I'm sobbing. Your son is precious and showed his love so openly."
The video has been viewed nearly four million times.
This picture taken on October 8, 2011 shows yoga instructor Suzette Ackermann (2nd L) and students taking part in a "doga" class, a yoga class for dogs, at a studio in Hong Kong. Hong Kong's pampered canines may have their own spas complete with jacuzzis and massage, but it can still be difficult for a dog to find inner peace. Each Saturday morning in the city's Sheung Wan district, owners massage their pets before bringing them into postures such as the cobra pose, in which the hind legs are stretched out to the rear, as soothing music plays. (DALE de la REY/AFP/Getty Images)
(02 of09)
Open Image Modal
A woman rests on the beach as a man rows his boat with his dog at sunset, 05 April 2007, during a festival at Nitzanim beach in southern Israel. The three-day festival, which is held during the Passover attracts many young Israelis, and includes rock concerts, meditation, yoga, and nudist camp. (MENAHEM KAHANA/AFP/Getty Images)
(03 of09)
Open Image Modal
A woman practicing meditation on the beach in easy pose, with a dog. (Getty Images)
(04 of09)
Open Image Modal
Raelene Mercer lies on her mat as her five-year-old dog, Montana, balances on her body during a "dog yoga" session at a local park in Miami Beach, Florida, 20 January, 2004. One afternoon of every month a small group of dog owners join yoga instructors to enjoy Yoga with their dogs. The "doga" or "ruff yoga" as it has come to be known was launched in New York. (ROBERTO SCHMIDT/AFP/Getty Images)
(05 of09)
Open Image Modal
Young woman, wearing a white body suit, is practising Hatha-Yoga outdoor between trees, showing the pose: vrischikasana, scorpion pose. A cat is looking at her. (Frank Bienewald/LightRocket via Getty Images)
(06 of09)
Open Image Modal
Lucky helps me balance, offers her trust and enthusiasm to fall into whatever adventure we set out for, like a true colie she is a great friend and purpose driven (Kristyna Djordjevicova)
(07 of09)
Open Image Modal
Lucy helps me garden....well I garden and she sits on my flowers with her "baby " . Lol (Patty Neiffer)
(08 of09)
Open Image Modal
Shy is my Border Collie mix and she has unlimited energy for any walk or hike I want to go on! (Allison Currie)
(09 of09)
Open Image Modal
This is Tesla. She and her friend Thor (a coonhound mix) have walked miles with me - about six a day on a country road in Michigan. Their enthusiasm always motivates regardless of the weather! (Donna Willard Kristensen)