I review (almost) weekly on my blog, and people send me a ton of unsolicited stuff. Some of it, I reject outright as not my thing (and of course, if the sample or blurb has any obvious issues, I decline to review) and some, I accept for review. Of those I accept for review, a small percentage get bailed on partway through (in which case, I generally write it up as a mini review, explaining why I bailed on the book). The rest will (eventually) get fully reviewed.
So, with that said, I currently have about 100 books in the queue, with an average of 3-5 a week getting added
When I am ready to write a review, I pick something at random based on my mood and what genre/book looks interesting at the moment. With that many books coming into the pipeline, there is no way I can keep track of who sent what, when. I tell authors I am happy to write a review, but they may have to wait awhile
As for general readers who don't blog about this, I don't think many of them feel any sort of obligation to review or not. I am thinking of people like my parents. I doubt my mother has ever written a review in her life, and I don't think it's ever crossed her mind that people at Amazon or wherever will care what she thought of the book. If an author is selling to her expecting that she'll do something back for them, they'll be waiting a long time