I do not know that much about Prolog. Only used it once or twice so far.
Would the internal facts database as storage be as efficient as a typical database, say SQLite or Postgres? What I mean by "efficient" is: Would it consume similar amounts of disk space or memory and would it be as fast in responding to queries with results?
Potentially yes, but current prolog implementations are mostly academic productions with few man-hours invested into them compared to more popular languages. So no, you won't build huge industrial websites on current prolog, but mid-size webapps are certainly within reach as modern prologs are much faster than people usually suspect.
Prolog huge data munging is a thing though. Check the "semantic web" and related for examples.
Would the internal facts database as storage be as efficient as a typical database, say SQLite or Postgres? What I mean by "efficient" is: Would it consume similar amounts of disk space or memory and would it be as fast in responding to queries with results?