Tie-in websites refers to websites written by Joseph Lidster and produced by the BBC, and a Twitter feed created by Hartswood Films. They tie into the show by creating a website or blog mentioned within the series.
The Science Of Deduction
The Science of Deduction is designed to look like Sherlock Holmes' personal website where he talks about his skills of deduction. The site was first mentioned in the show by John who said he had looked Sherlock up and had found and read the website. The site plays a big part of the plot in "The Great Game" as Sherlock uses it in order to signal that he has solved each of the pip puzzles and where he signals Jim Moriarty to meet him at the pool.
The tie in website contains case files on the cases which made up episodes of the show as well as other off screen cases. The site also has a "Hidden messages" section where an unknown person (hinted to be Moriarty) is sending Sherlock coded messages.
The personal blog of Dr John Watson
John's blog (also known as Watson's blog) is a website created by the BBC which is a copy of John's blog entries that are mentioned or seen in Sherlock, along with comments from characters in the series. The blog also talks about cases not seen or mentioned in the series or cases, such as the small cases in "A Scandal in Belgravia" that weren't explained.
In the blog John, on the advice by his therapist, writes his thoughts and feelings about returning to civilian life. He does not start regularly updating until "A Study in Pink" when he first meets Sherlock.
In "The Great Game" we find out that John has written a detailed account of the events of "A Study in Pink" which Sherlock disapproves of but is read by DI Greg Lestrade and the other members of the London police. By the time of "A Scandal in Belgravia", the blog has become an internet phenomenon and is viewed by nearly 2000 people in one day. These fans have even started showing up at crime scenes in order to catch a glimpse of the duo with several newspapers reporting on this, Sherlock and John have appeared in newspapers.
The counter of John's blog is currently stuck at "1895" despite John trying to reset it. Sherlock thought this meant the blog had been locked like this in order to give him the password to Irene Adler's phone. The password was not accepted and it currently is unknown what happened to the counter. It was hinted in the director commentary for "A Scandal in Belgravia" that the hit counter was merely a joke, as 1895 was the year in which Sherlock Holmes was at his prime, according to the original stories by Conan Doyle.
Connie Prince – Beauty Queen of Hearts
@Sherlock221B is the official BBC twitter account for Sherlock Holmes, but is mainly used for promotional purposes (not much original material). ContactSH is a more personal, casual, and "real", though unofficial, Sherlock Twitter account.