Richard Branson uses a .IN domain hack for his brand name Virgin, and his LinkedIN posts show virg.in as the source. Clicking on virg.in takes us to his blog. See image below.
LinkedIN.com owns Linked.in. Not "they" (which contextually would be the Virgin group).
And, I wouldn't really call linked.in a hack. It's a trademark defensive registration forwarding to linkedin.com.
Hacks would be words where "in" is part of a dictionary word. For example:
Virgin
Bulletin
Coin (but, LL.in is not allowed by the registry, minimum is 3L).
You can check Words that end with In, words ending with In for valid words and maybe some ideas for new domains
Incidentally, none of the possible 3Ls from above list is available.