|The demon that stops us getting to Mars|
A publisher means you should have a page for your book/professional life, not a normal Facebook account. So - you join Facebook, you friend your friends and your family and some randomer you met on a train and your plumber, bla bla. And you post your holiday photos and links to (damnit) stupid kitten photos. But you don't friend your readers.
Please, please, don't friend your readers, especially if you write for children/young people. Because then you have to be even more careful about what you put on FB, and set up groups to keep the young readers separate from your posts about how you went on a disastrous date or you accidentally dyed your hair blue, or were sick after having too many cocktails. And don't friend your editors or your agent or your publicist (dream on) and a load of librarians and booksellers unless you are going to put time into managing lists so that you can control who sees what.
Make sure you manage your security settings so that all your private exploits are not visible to the public. It's not hard, and it's the online equivalent of not leaving your door open when you go out. And watch out for other people tagging you in photos of disreputable behaviour. Not that you engage in disreputable behaviour, of course. But if you did, untag yourself.
Once you have an account, create a page - this is the professional bit. You can create a page for you-as-author or for one of your books/series/characters. Scroll right down to the bottom of your Facebook screen and there is a link 'Create a Page'. Follow it; do as it says - I'm not doing a tutorial on creating pages (unless you really want one). On your page you can share stuff that is relevant to your book or writing life, such as links to reviews, info on events you are doing, web pages that might be interesting to your readers and so on.
People who like your work can find your page if they search for you on Facebook and can 'Like' it. You can control what they can do on your page, but it's nice to let them leave comments. Think carefully about how you are going to use your page. Are you just going to put news on it (new books, events, etc)? Or are you going to be more chatty, and put updates on your writing, projects, thoughts, blog posts, etc? Decide on your strategy and stick to it. The news approach is less work for you - but less interesting for your readers.
Your publisher will hope you will get lots of people liking your page and looking on it for information so that they can rush out and buy your next book. It's tempting to try to get lots of Likes to endorse yourself and make you feel as though you weren't wasting your time making the page. Even so, I hope you won't spam all your friends asking them to 'Like' your page. I have a strong dislike of being asked to 'like' pages for things I've not read. If you want to say you have a page, that's OK, but please don't ask me to like it. That's rather like saying - 'tomorrow is my birthday. Please buy me a present.'
From your author/book page, you can like other pages. Do it thoughtfully. If your books are for 9-year-olds, don't 'like' a page for gin or serial-killer fiction. So if you write undersea adventures, you might like pages such as 'shipwrecks' or 'sea mysteries' (I have no idea if those exist, but you get the idea). You can pretend. You could even pretend to like pages about silly kitten photos if that would appeal to your readers and fit in with your books. You can like bookshops and libraries from here with no danger of the manager of the local Waterstone[']s turning against you because you have slated their favourite holiday destination or had a row with their cousin - they can't see your personal page (as long as it's not public).
Get your publisher to 'Like' your page - it's their job to promote you. There is an important distinction here. Don't ask your friends to 'like' your page - they are your personal contacts. Do ask your professional partners to 'like' your page - your publisher/agent/local bookstore. Not other writers. We are not your publicists; we will like your page if we want to. Those of us who write reviews will probably not like your page, even if we like your work, as we might see it as a public compromise of our impartiality.
Put a link to your page in your email signature, put it on your website, your blog, your twitter profile page, your business cards (if you still have those), bookmarks and other freebies, and get the publisher to put the link on the imprint page of your books and on their publicity. After all, if they want you to do this stuff, they should support it.
If you don't want to do the kitten-sharing, cousin-hugging bit of Facebook, you can create an account and then ignore it, though you will have to collect a few friends before you are allowed to make an author page. [Purple bit changed - thanks to Mary Hoffman for the correction.] To stop people trying to friend you (they will find you, sooner or later), you can make yourself undiscoverable. Which is perhaps the ideal - you can be a as private as you like, and have the digital equivalent of a cardboard cut-out you that trollops around Facebook.