Whenever I write a message, I've thought about the person in question quite a bit and considered the best way to talk one-on-one based on what we know about each other. I also take the time out of my day to write something with meaning so that it's not just a random short message or spam (like chain letters). FB has a thing where they show a time stamp of when your friend read the message. Whenever I see that they've read it and don't write back for a while, but they have time to post updates, comment on status updates and pictures, and do other stuff, I'm not sure about our relationship anymore.
Maybe they are so quick to assume that private messages are filled with spam or something, but when it's from an actual human being, it would be nice if they replied.
I understand that sometimes it involves a topic where they don't feel comfortable replying because it's not something they wish to discuss or because they feel that there's nothing they can say to help. They could at least write back and say they can't help me out so I can stop putting other things on hold in case my friend can help me out. I don't want to make multiple plans and run into conflicts later where I have to make difficult decisions.
What can I do at this point? "Take a hint" and assume this friend doesn't want to talk to me? Some people have time to quickly read a message but they don't have time to sit and write back, and I get that. But when they don't write back for a week or a month (happened to me with several people, at the same school!), what am I supposed to think? We're supposed to pick up on clues from other people these days where we leave people alone if they just don't reply to anything we say.