Managing a social media presence can also take up a lot of time, and it's disruptive of attention - you can flit in and out of Twitter but that doesn't help you sustain concentration as you try to write your novel. In fact, it can make it increasingly harder to concentrate as you 'lose the habit' of writing fiction in favour of developing the habit of tweeting. For that reason, too, I'm not that fussed about authors having a social media presence before I take them on.
We also have no data to prove that social media presence leads to increased book sales for any author who does not get a contract because of their social media presence - that is, if you're an author who has become famous for your blog, we can expect that your blog will play a large part in the promotion of your book, but if you're an author who has written a great novel, is having a blog going to help all that much? We just don't know. So I'm loath to tell authors to spend a lot of time blogging when it could be disruptive of their attention and consume a lot of their time without any discernible benefit to their writing career.
Of course, we may get data soon - and if it's demonstrated that social media is important to some or all all types of writers, then those types of writers will need to engage with it. But if we do get that proof that social media is so important in the promotion of the book, that's actually something the publisher should provide a lot of assistance with - to the point of undertaking to do all the social media for authors who simply aren't great at it. And some aren't - their Twitter voice isn't engaging, or all they do is implore people to buy their books. Those authors should stay away from social media - but if they absolutely need to have a social media presence, the publisher should help them with it rather than let them hoist themselves on their own petard.
And here is where we start to look at the suite of services publishers can offer writers - and at whether or not 'new contracts' should start to accommodate them. If one author is great at social media and can handle everything themselves, should they receive the same royalty as someone who needs a publicist to do it all? It's entirely plausible that the social-media-adept author might start to think there should be some allowance made for what is, in effect, an extra skill that they're bringing to the publishing contract. And what happens if that particular author has to choose between publishers - one publisher offering them a slightly higher royalty rate in recognition of this skill, and the other not? Will this start to become a factor in the author's decision making? Perhaps not. But perhaps it will.
In the meantime, though, my advice would be to focus on your writing. I'm sure I'm not the only agent in the world who is more interested in how an author writes their manuscript than how they write their tweets.