Lev Grossman's The Magician King has a neat subplot revolving around his character Julia that heavily involves an internet forum and the friends she makes there.
It's not the main plot of the book, but it's quite interesting. Might be worth checking out. However, The Magician King is the sequel to The Magicians and probably shouldn't be read out of order. Great books, though, totally worth reading.
Also, maybe check out Silently and Very Fast by Catherynne M. Valente. It's a sci-fi novella that's set a bit in the future. It revolves around the main character's relationship with an AI program. However, the author has some really interesting ideas about how the internet and technology will evolve.