At the moment, only Facebook and Twitter is supported, with plans to support everything that the Dukt OAuth plugin (opens new window) does.
Fair enough - we totally understand its a little weird giving over control to your social media channels to a plugin such as this. We would encourage you to use this responsibly and have your accounts set to disabled by default, then you won't accidentally post something to social media. In this sense, its much for of an opt-in per-entry.
Not to worry, once an entry has been posted to social media successfully, posting is automatically disabled. You have to actively enable each account to post again, so there's no risk of it appearing multiple times.
Posts to social media will also only ever occur if the entry is set to live. This means if you disable the entry, set its publish date in the future, or save a draft - none of these actions will trigger a post to social media.