How do users know which site to post on?
I think a site-wide search may help. EG: I'd type a word in the search box, and the return would include sites that have tags for that term, or questions for that term in the title, or questions that have that term in the body.
This is especially important now with a wide range of sites, and some overlap between a few of the sites.
I saw this question: What do I do if I don't know where to ask my question? which has some sort of answers.