I've picked up a lot of freebies (both for myself, and just for my personal records of the official publisher promo things offered so I know what's a repeat) and have read considerably less than 1% of just the publisher promo stuff.
But for those things which I did read and liked, I have a very good follow-up buy-further-in-series/other-items-from-author-or-imprint ratio of somewhere in the neighbourhood of 60%+, going by my previous years' read lists in the Reading Recs forums.
Of course, I'm fairly picky about what I actually do start to read and finish (allowing for the occasional indulgence in total crackfic), so that probably accounts for some of it, as I tend to choose reading freebies which I'd normally have reasonably high interest in if they showed up as library books.
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