Marilyn Johnson wrote: They...don't seem to stick around for long. I don't know if it's because of lack of reviews, or because the reviews they get don't live up to the expectations they've put on their work. But I doubt any new folks will get any reviews if they don't post in something that pays points. Sometimes I log on and get the feeling that I am the only person in the world on here. It's a strange feeling.
How about that, J P? I didn't bash the president (though he needs it!). 
Don't worry about the bashing. I'm used to it now.
About the disappearing members--what you say about points as an incentive to review is probably correct. I remember being stunned after taking the time to write reviews and not getting my Pavlovian reward. This happened a few times before I caught on (Pavlov was right). I still get caught from time to time, but that's not my main criterion for reviewing.
If I spot a work that has an offensive (this is purely subjective), boring, or ungrammatical title, I don't read it. If after skimming the first few paragraphs, I find the substance to be any of the three above, I don't review it. Then if, after reviewing the first few chapters, the author still shows no sign of learning the difference between 'then' and 'than,' 'its' and 'it's,' or when to say, 'Joe and me,' rather than 'Joe and I,' I write them off.
So I think maybe your second reason is more likely, which is why we don't ask our friends and relatives to review our work. Friends and relatives, sharing the same environment, have the same level of literary expertise. After being surrounded by people who praise our writing, it's tough to take the hard truth. I know I quit the site after only a couple of months of folks telling me I was 'head-hopping,' or 'telling rather than showing,' or the most damning, my 'character showed no signs of problem-solving.' The reason? I was too dumb to know what any of that meant!
I tried a few other sites, and they were totally worthless--just a few kids bragging about their social life and writing new episodes of Star Trek. So I came back, and with the help of a few thoughtful reviewers, I have learned a lot. Now when I write a review, if I feel the writer won't understand my critique, I explain why I'm saying something, and may even give them links to online sources. This is more helpful than throwing buzz words at them. And the people who really want to learn how to write will come back.
But you know what? Life happens, things change, and humans adapt. There are any number of reasons why our friends have not been active, including family tragedies and grave illness, even death. There are positive reasons, too--when your writing takes off and you're going on book tours and appearing on the Today Show, can you really say you'll still be logging in to tnbw faithfully? Or maybe you'll hit the lottery and decide to travel the world. Will you still have time for the rest of us then?
Stick with those of us who are still here and don't worry about the rest--they'll either come back, or they won't.
JP