Each Stack Exchange site has an associated Twitter account that is set up by the company when the site's public beta begins. These all follow the naming convention @Stack[topic] - more or less. So, to answer your question, the @StackSciFi account was created by the company, and it, like almost all of our Twitter accounts, tweets content that is determined by an algorithm. I believe the most recent version (as explained by Jeff Atwood on MSO) is:
- per-site blog posts (if present)
- chat events, if started by a moderator
- interesting questions on the site
Every three hours (on average).
- bountied questions
- interesting answers of +3 or better
- unanswered questions of +3 or better
- particularly hot meta questions (not support or bug though)
About every two hours.
Anything that has a mention in the edit log of being promoted via Twitter has been tweeted by one of our bots.
In addition to the per-site twitter accounts, there is also an account for the whole network: @StackExchange. This is also managed by an algorithm that has it tweet out interesting questions from around the network, similar to the way stackexchange.com/questions works.
On a slightly different note, there are a handful of accounts that are run by the SE team. The @StackHQ twitter account is sometimes used to share questions and answers, though we tend to use it more for office antics, contests we're running, or other fun news.
There are also Stack Exchange Facebook and Google Plus pages that promote content and news from around the network. These are both run by humans, not bots. If you have Scifi & Fantasy posts that you think are especially worthy of promotion to a wider audience, you can submit them and we'll try to post them.