Just about any actor you can think of has fans that make “fan videos” based on clips from the actor’s movies or TV shows. Then other fans watch them and everybody comments and shares the videos. Some times the fans go onto the “celebrities” Twitter or Facebook page to let them know about the videos that they like. Some times the celebrities will Tweet about a fan video that was made for them, and it goes back and forth.
There’s tons of good fan videos that have been made for all of the actors in the Harry Potter movies on youtube.
There’s also “fan fiction” in which fans write their own stories as though they were “real” TV episodes, or new movies. There are sites dedicated specifically for fan fiction. And there are more adult versions of this that are often called “slash” in which sexual situations are thrown into the mix. These stories are shared amongst the fans and are also put up on the actor’s Facebook page.
Some of the authors that I like have websites and you can go on there and participate in a discussion about their books with other readers and sometimes the author joins in the discussion. All of these things can be “liked” on a Facebook page so that your social network friends can go there and check it out.
Then there are websites that are about a particular celebrity that might promote a “fan gathering” or “fan convention” like Beatlefest or Comic Con or a Star Trek Convention. The managers of the website, get the word out and then the fans themselves spread the info by e-mail and Facebook and Twitter.
The celebrities who have their own websites, often have a day or a time, that they announce ahead of time where the fans can go on and have live discussions with them too.