The 33-chapter series is called Lost on a Red Minibus to Tai Po and it's about a bunch of passengers who are lost in a parallel world - where the surroundings remain the same, yet all humans have disappeared.
In July, the online version was edited into a book and it became one of the three bestsellers at this year's Hong Kong Book Fair.
"I started writing Red Van when I was on holiday in a hotel in Japan," says Pizza, who won't reveal his real name because he doesn't want his humble writing "stardom" to affect his personal life.
Red Van is how the young writer, 23, refers to his sci-fi fiction in Cantonese.
"After I graduated with a film degree in Britain, I moved back to Hong Kong," Pizza says. "Then, I started writing scripts for some short films shot on the mainland."
Although he wrote scripts for six months, he was never paid.
"I was so frustrated during that period. Then I thought I shouldn't rely on other people any more, but myself."
A big fan of American writer Stephen King - who is a master of horror, suspense and science-fiction stories - and the US television series Lost, Pizza decided to create his own sci-fi with a local touch. He wrote Red Van in a conversational way, using local slang to relate to the online community. Then, he clicked submit and posted it on Golden Forum.
Pizza's tale of thrills and suspense soon attracted a loyal following, but he didn't expect that it would be a huge success.
He found that he was forced to make an extra effort to maintain a high standard of work. "Initially, I thought it'd be similar to writing normal fiction," he says. "But it's actually very exhausting because you have to produce constant content and appeal to your readers."
Feedback and criticism can be welcomed by writers, but the anonymous nature of the internet encourages strangers to leave unreasonable and nasty comments without being responsible for what they say. It took a while for Pizza to get used to it.
"If you write stories or columns for newspapers and magazines, you don't have to interact with your readers directly," Pizza says. "The worst thing that can happen is that they write a letter of complaint to your editor or publisher."
But in the internet world, writers have to come face to face with their critics.
When Pizza got some hateful comments, he described it as "like you are writing something in your room, but somehow someone barges in and shouts at you in your face".
Even after he knew that his book would be published, he still had to face random internet users who accused him of being only in it for the money, and urged other Golden Forum users not to buy his book.
"I don't understand why these people had to make up stuff, even though they could read the entire story online without spending a dollar," he says.
Against all odds, the book, which contains adult themes and strong language, went on sale at the book fair, and has sold 30,000 copies so far.
If he was to do it all over again, Pizza would be more prepared. He said he would finish the whole book before uploading it onto the web.
However, what Pizza really wants is to get into the local film industry.
Despite his plan for a second fiction series, he sees writing as a stepping stone to becoming a good director in the future.
"To learn how to tell a story ... writing fiction is good practice," he says.
But to be able to make a film, it takes someone to be on a set and to observe. Pizza is sure he has what it takes to take the big step. "I'm ready."