I've been told that most recent loanwords from English are masculine (e.g., week-end, show, internet, muffin, software, match, sweepstake, blog): could someone please provide a reputable reference substantiating this claim? In particular, and as an example, is the gender of "Facebook" and "Bitcoin" masculine? Finally, are there many notable exceptions to the assignment of male gender (such as the word "la star")?
I've found a quite comprehensive list on the wiktionnaire https://fr.wiktionary.org/wiki/Annexe:Mots_fran%C3%A7ais_d%E2%80%99origine_anglaise and it is a fact that nearly all the loanwords from English to French are masculine.
Note that words that are not modified (francisés) seem to all be masculine. There are very few exceptions otherwise and they are often obvious: la/le babysitter, la pompom girl, la miss or with a spelling that make them sound like a French word: la fission (sounds like mission, session, dépression...), une overdose (la dose in French), une motion, la randomisation, la standing ovation (une attention, une émotion...).
@SGimenez mentioned two more exceptions in his comment, which are not included in the rules : la start-up (une société) and la soul (une musique). And I would add la rocking-chair (une chaise) to this list of exception, as well as la star and la battle mentioned earlier.
Reddit features many more posts on this subject, too numerous to be reproduced here, but I just quote a few comments from this chain:
In French if a word is borrowed directly from English (un anglicisme) it will be masculine. If the borrowed word is changed to a frenchified spelling and pronunciation, it could be masculine or feminine based on the spelling or what have you. For example, all words in French that end in -tion, -sion, -ure, and -ité are feminine. La torsion, la structure, la fraternité.
I'm going to have to disagree here. There is a strong tendency in French to assign masculine gender to borrowings from languages without gender. With the exception of some well-established anglicisms in Quebec French, the only time we seem to find feminine borrowings is when there is a corresponding native feminine lemma. I can't think of any recent French borrowings that fill a semantic gap (i.e. whose denotatum had no word for it, e.g. le sushi, le zouk) that are feminine.