Listed as 'shagbook' in a vaguely but not quite 'facebook'-like font, and in two colours, the text also names the site as 'Shag Book' to confuse things up a bit.
A disclaimer in the bottom right hand corner of the site's sign-up page states: 'All models are age 18 or older. The Shag Book ® dating site is meant for users aged 18 or older. If you are not age 18, or older, please leave this site now. Shag Book ® is not affiliated with Facebook, Facebook of Sex, F---book, or any other site with "Book" in its name.'
Unfortunately for Shagbook, this hasn't stopped Facebook from taking action to protect its trademark, according to Mashable's detailed 'Facebook vs Shagbook' article.
Facebook claims it would be damaged if Shagbook was successful in keeping the Shagbook name, with Shagbook striking back to state that the word 'facebook' is really a generic term and thus Facebook's claims have no validity.
Facebook predictably says words to the effect that confusion between the two along with the danger of any implied endorsement is grounds enough for Shagbook to be told to shag off instead and to leave Facebook alone.
Mashable quotes the company representing Shagbook in its filing, SNRG Ventures, that Shagbook's American owner 'referred to his little black book as his little 'Shagbook'' when he was 'living in the UK', and that 'he was amused with the word 'shag,' and picked up the name Shagbook.com, all perfectly innocently'.
Presumably Facebook will say that it's a likely story, but from a layman's perspective (no pun intended) it sounds more than plausible enough for me, with Austin Powers already having injected his shagadelicness into the popular culture long before Facebook was a glint in the Winklevoss twins' eyes and then re-glinted by the extremely trustworthy-in-business and vigourously avowed super-champion of privacy, Mark Zuckerberg.
Links to court filings and more detail here.