Though more of a question about promote yourself on social networks when that time comes. Is there any reason it has to be Twitter or Facebook specifically? I've recently found better social networks that are more private. So I've been cutting down my presence a bit on Twitter.
Or does it just have to be a social network?
Would you rather talk simultaneously to 1000 people or 10? How about 1010 people? The answer is you want to expose yourself and your work to as many places and as many ways as possible. Don't think of social networking as its own thing, think of it as simply one of the ways that you can get in front of potential buyers for your work, and I wouldn't dismiss live readings, one-on-one networking and media interviews as possibilities either.
Sometimes talking to 1 person is better than promoting your work to 1000 because the 1 person may be likely to buy your book while the 1000 don't because you fail to make a personal connection.