SAN FRANCISCO (CBS SF) — Boo, the tiny Pomeranian who shot to Internet fame and delighted millions of fans daily with his playful antics, has died.

Boo’s owner wrote a heartfelt message to the dog’s 16 million Facebook followers saying the tiny dog passed away in his sleep on Friday morning. He was 12 years old.

In September 2017, his playmate Buddy died at the age of 14 and ever since, the letter says, Boo began having issues with his heart.

“We think his heart literally broke when Buddy left us. He hung on and gave us over a year. But it looks like it was his time, and I’m sure it was a most joyous moment for them when they saw each other in heaven,” said the letter, signed ‘Boo’s humans.’

Buddy was also a Pomeranian, but his coat was left long and silky.

Boo sported a buzz cut that made him look like a little lion cub. If imitation is the sincerest form of flattery, Boo had many admirers — Poms sporting lion cuts and tiny tuxedos over their shorn bodies — around the world.

Photos of Boo and Buddy first went viral ten years ago, when their owner began posting pictures of the two along with their towering sidekick, a Golden Retriever named Blue.

Familiar Bay Area bridges sometimes provided a backdrop for many of their photos.

In 2012, a report saying that Boo’s owner was a Facebook employee named Irene Ahn, did nothing to dampen the little dog’s popularity.

The owner never entered the spotlight. In addition to posting his many hundreds of photos on Facebook and Instagram, under a pseudonym, Ahn published Boo’s 2011 book titled, “Boo: The Life of the World’s Cutest Dog.”

The letter announcing Boo’s passing has prompted an outpouring of sympathy. So far, more than 49,000 people have written saying the news brought them to tears, and expressing gratitude for a decade of friendship with the cutest dog they’d ever seen.

It seems Boo was everyone’s friend. Perhaps now he is reunited with his.

“We know that Buddy was the first to greet him on the other side of that rainbow bridge, and this is likely the most excited either of them have been in a long time,” said the letter. “Boo, we love you with all our hearts and will miss you until the day we meet again. Have fun running around with Buddy and creating adorable mischief wherever you guys go.”