Can digital copies be shared or borrowed like old fashioned books?
You can either do the library method, or find them through other "sources"
Sure you can. Just takes some creative activities.
This is why I stayed away from Kindle, the Kobo and other readers allow for much more variety in terms of files (ePub, doc, pdf, etc.) whereas Kindle has their own ecosystem and are trickier.
No different than the old days of sharing music / video files. Kobo FTW in this aspect. Much easier to share.
I couldn't believe it the first time I had to 'wait' to loan a book from TPL. Like WTF...it's a digital copy! Apparently they only have X licenses, and those are loaned out same as physical books.
Kindle now supports ePub by default, they are moving away from the Kindle Mobi proprietary format.