SEOUL, Dec 23 (Reuters) - When Jin became the first member of K-Pop supergroup BTS to enlist for South Korea's mandatory military service this month, he had a special message to fans on Weverse, a fan platform app that commands more than 8 million active users.

"Now it's curtain call time (I wanted to say this when I go to military service)," said Jin. Devoted fans posted more than 10,000 replies.

The Weverse platform has proven a major weapon for the K-Pop group's agency, HYBE Co Ltd, offering fan-oriented experiences interacting with the stars, accessing unique content and buying merchandise. It is now expanding into subscription services and more.

"We are a fandom business," Weverse President Joon Choi told Reuters. "There are bigger global services offering functions we offer, but Weverse's users are superfans characterised by passionate engagement."

Before Weverse launched in 2019, HYBE artists' fans were scattered across multiple platforms, said Choi.

"They bought merch here, watched videos there, communicated elsewhere... We didn't have a database of our customers. So we began developing each service in-house."

The K-Pop industry has revolved around loyalty and personal identification with the stars, helping to cushion it against the impact of economic downturns. Revenue is driven by public response to artists' activities, and upselling is possible, analysts said.

HYBE is now uniquely positioned to harness such a loyal fan base to grow its global business through unit Weverse's technological reach, offering a growth model for future entertainment, analysts said.

Around 170 out of 300 Weverse employees are engineers including programmers and UX specialists, Choi said, with veterans from gaming companies or local tech giant Naver with which it has a partnership.

The app has about 80 teams of artists represented including BTS, and about 8-9 million monthly active users. It also has a high number of repeat visitors and retention.

Non-HYBE artists are also on the platform, including rival agency YG's top girl group Blackpink.

More U.S. and Japanese artists are expected to join next year, said Choi, declining to reveal who.

HYBE acquired Ithaca Holdings, representing Ariana Grande and Justin Bieber, in a $1.05 billion deal in 2021.

Choi, who worked at gaming company Nexon and "Baby Shark" content creator Pinkfong before joining Weverse, says the app has added services such as live streaming, Instagram Story-style updates, free and paid content, artist-to-fan interactions and merchandise retailing.

Weverse users' 'superfan' qualities make engagement in those services turbo-charged.

"For example, we do commerce, but there is no seasonality... We have 80 teams of artists, and some of them are always doing something," he said.

App users are based in more than 200 countries, notably Japan and the United States, and only about 10% speak Korean, Choi said.

"There is no other entertainment agency with such a fan platform with enough users for economy of scale," said Lee Hye-in, analyst at Yuanta Securities. "...In K-Pop, the agency holds the hegemony."

Shares in HYBE closed flat on Friday. They have risen about 64% from an all-time low hit in October. (Reporting by Joyce Lee; Editing by Michael Perry)