Does an author have to be on every social media site?

The best way for authors to get ahead is to concentrate on their product -- i.e writing the next book and making sure it is the best that it can possibly be. You can't sell anything if your shop window is empty. However, more than ever there is  the siren call of social media. Since I started writing seriously, there  any number of social media platforms -- Facebook, Goodreads, twitter, myspace, blogging have sprung up eharlequin etc.  Now kindle social, a.noblii and google plus, furthermore there are places springing up
The clarion cry goes  up a la the 1849 California Gold Rush, they are bringing up gold nuggets the size of boulders in Red Dog Flats! And the poor romance author wonders if she should be there as well.So also looks at the clock, the deadline she signed in blood and her family and her heart sinks.
Some respected social media people like Kristen Lamb -- say that you don't have to be EVERYWHERE. (Btw her book We Are Not Alone is a very good primer for authors..even if it does need a bit of updating) However, I think the statement needs slight tweaking. An author doesn't need to be everywhere but the ability to contact her does.  An author needs to be contactable by her readers. It is why one of the most important things for an author is her website (yes I know mine needs updating...) It is about allowing readers to contact you using their chosen medium.
So you don't have to everywhere but it is no reason why you can't make use of the RSS (Really Simple Syndication) that is embedded in many things.
For example, when I post this blog to blogger, through the magic of RSS, it also goes out  Facebook, twitter, Amazon.com, Amazon.co.uk. Amazon.fr and Good Reads and possibly one or two other ones that I have forgotten. I know that it is part of the British Library Internet preservation programme for example.  The main point of the exercise is to allow people to read my blog using THEIR chosen medium, rather than mine.
I also enable comments and get notifications via the email. I do try to response to all comments wherever they are posted. I really value my readers. I also value the friendships that I have developed through modern technology.
So where can you find me? Hopefully everywhere, but I also use modern technology to be there and be able to write at the same time.
Eharlequin should hopefully get RSS capability soon btw. At the moment, I want to post there I do have to do a cut and paste thing.Time consuming) I love the eharlequin (and the Mills and Boon) boards. They helped to keep me sane when I was unpublished and still maintain that function these days. I enjoy Good reads, particularly its never ending book quiz. Good reads giveaways are brilliant btw. And shelfari's end notes are linked to Amazon so that when a person downloads one of my books via kindle, they can get additional info (some of this is an ongoing project as it is a labour of love)
I do twitter (@MichelleLStyles) and do follow people back as I am interested. I go on FB. I have started sharing music that inspires me on myspace (and no it does not start playing)  I love interacting with people but mainly I try to remember that my job is to write books and that I am in the midst of a multi-book contract where once I deliver those books, they will eventually be in the hands of readers the world over. Therefore I make no apologies for using RSS and other modern linking devices.
So work smarter, not harder but allow the public to find you in whatever social media platform they chose.