To my surprise I was listed as a top publishing industry tweep (aka Twitter person) by The Bookseller even though I don't think I'm much good at networking and am hopeless at technology. This makes me think that if I can do it, anybody can.
Twitter is best thought of along the same lines as a drinks party. At this party it's socially acceptable to eavesdrop on conversations and join in if you've something to say even if you don't know the people talking, but generally the party operates on the usual lines: only the most socially inept people bang on about themselves all the time, conversations are about give and take, and no one likes being sold things at a social event.
At this drinks party you wouldn't suddenly tell people you'd just met all about your private life, nor would you feel obliged to speak to absolutely everyone present. And you certainly wouldn't badger people just because you thought they might be useful for your career.
The sort of things you'd talk about would be things you thought might be either amusing or informative. You might indulge in a bit of moaning about, say, a tradesman who hadn't turned up when they said they would or if things weren't going brilliantly, but you wouldn't start ranting or being mean.
In general, behave as you would at any real-life social event and you won't go far wrong.
Blogging is different. With that you have to think about what you offer a potential reader, such as entertainment or information. Blogs that are basically Me Me Me don't pick up many followers unless 'Me' already has a following. I think it's quite hard for a writer to have an interesting blog unless they blog about craft because what else is there apart from your personal life? I'm not happy with blogging about my own life - and frankly, it would make for dull reading as I don't do much except read, write, teach and trundle up and down the A30.
If I was unpublished so didn't feel confident about dishing out advice about craft (not that that stops some writers) I would concentrate on the business of publishing from a new writer's perspective and book reviews.
What I don't think blogging or Twitter or Facebook do is sell books, at least not directly. They are about networking - building up contacts with people who are active in publishing (or whatever industry you choose to get involved with), which in turn raises your profile and that may lead to other things such as speaking at events or free books (especially if you do book reviews), or may help to find an agent or get an article published.
I started blogging and being on Twitter because my publisher told me I 'had' to, and then I found I liked it so have carried on. If I didn't find it enjoyable I would have quietly drifted away. There are lots of authors who have very successful careers without bothering and if I had a choice between writing a good book or spending time on social media, writing a good book would win out every time.