I don't care what other writers do, but for myself, I don't want to share my numbers, and here's why.
First, I value my privacy. I don't tell people on the Internet where I live, I don't give out my phone number to strangers, and I don't leave the curtains open when I get undressed.
Second, I want the interest to be focused on my books, and not on how many copies my books sell. I'd rather have people say, "That character is really compelling" than "I can't believe she sold (or only sold) x copies last month."
Third, I see only limited benefits in sharing sales figures. Yes, if my books are selling really well, this fact might inspire others. However, I think it's equally possible that if my books are selling well, this fact might discourage others whose books are not selling as well. Different people react differently to others' success. Some applaud it and some are encouraged by it; but some are depressed by it, some disdain it, and some feel a need to disparage it. In my experience, people in general--especially on the Internet--are more likely to deride someone else's success than to offer congratulations. On the other hand, if my books are selling poorly, this fact might discourage other indie writers. And it might cause potential readers to be dismissive: "If the books are so good, why aren't they selling?"
Again, I can't--and don't want to--tell other writers what to do. But for myself, I don't believe in sharing my numbers.
Of course, that might change when I've sold a million books!