I would think very carefully about what you want to achieve by auto posting, and what level of upkeep you want to invest in this. Do you really want everything going to every one ?
Think of things that can go wrong (post a typo, then repost it, update all your articles - all the posts re-broadcast, apis change - I've seen them all).
What I would probably do in the first instance is look at something that polls your site, and posts for you, rather than having it sit within a site. That takes away some of the upkeep of apis and the like.
Zapier is excellent for this - RSS by Zapier lists a number of the possible integrations by creating an RSS feed on your site and having Zapier use that.
IFTTT is also excellent for this.
I'd also add buffer into the mix that handles timing of pushing content cleverly, so you push it at a good time, when it's likely to be read.
These can all post to all the networks for you, and add many more features besides.
To achieve what you want, my suggested approach would be something like the following:
- create a view for published content you want to post, add a field (defaulting to true if you'd like) to specify include.
- set up an integration with either IFTTT or Zapier to poll these feeds.
- set that integration up to push these items into buffer and have buffer manage the posting for you.
I know it's not a sexy code based solution - but I'm of the opinion just because you can do it with Drupal, you shouldn't also look at what else has been done, and how others achieve this kind of thing (getting off the island).
This way is scalable, reliable - takes away a lot of the integration pain, and in my experience works really well.