If I understand correctly, there are two approaches to pronouncing borrowed, non-transliterated words: either err on the side of approximating the foreign pronunciation, or adjust the pronunciation so that it sounds natural in Mandarin.
For example, "Facebook" can be either
[feis buk]. This is not really valid pinyin, it violates Mandarin phonotactics, but nevertheless I heard it in the audio materials for a learning app, so it must be a thing.
[fei si bu ke] with various possible choices of tones. E.g., the humorous 非死不可.
So do native speakers prefer to violate Mandarine phonotactics for such words, or do they try to shoehorn the foreign pronunciation into Mandarin's phonetic inventory?
Does the choice depend on how well the speaker knows English? Is one pronunciation considered more fashionable than the other?
Also, are there common borrowed words for which the first type of pronunciation (violating the Mandarin phonotactics) is the overwhelmingly preferred choice?
(I know people often say 臉書 etc., but I am wondering specifically about the cases when the borrowed spelling "Facebook" is used).