Wired have a nice write-up about the team behind LonelyGirl15, looking at the background and possible future. The article mentions a few things that helped the success along, how they responded to the fans and how they managed the revelation that Bree was not ‘real’.

• they kept within legal bounds by not trying to sell anything related with the character and by never lying directly. The legal advice if someone asked whether she was real at the beginning was not to answer.
• they launched by responding to a video from Emily, who already had a following. The short video was creative enough to generate a lot of response and a call for Bree to post more.
• they built up an audience slowly, changing the story based on responses and ensured they established a presence int he community. A key insight was gained from their first ‘hit’, a video where Bree showed a lot of emotion. “Viewers wanted family and relationship drama mixed with a rich, mysterious backstory that could be explored and debated.”
• they provided mystery and the opportunity to explore – who was she, where did she live, what religion where her parents. The videos encouraged speculation and commentary.
• they hired someone to respond to emails full time (they are currently answering about 500/day). By asking about the emailers lives, they invited a conversation
• following the news that Bree was a fictional character there was some backlash and a lot of press interest. But they have continued the story, maintaining a healthy level of viewers for the series, generating income through advertising on their site.
• next steps involve creating a mystery, allowing fans to continue to speculate and comment, in the way that Lost does. They’re extending the world, producing content by other inhabitants, letting fans follow their own path through the information

There’s some key lessons here that content producers, including advertisers, can take away to apply to their transmedia properties. Interaction, conversation, letting fans run away with the content are all important in building an interest.