I am not talking about contacting a user about a question/answer on a site. It's just for other information or asking for some personal advice, or just commenting a user on his/her 'about' profile! How can I leave a message for a user?
You can't, at least within the boundaries of Stack Exchange, and this is by design. It doesn't offer a private message system, some of the reasons being that Stack Exchange's focus is the content, not the users themselves; also, private communication would be hard to moderate.
Users who don't mind being contacted sometimes leave an email address on their profile page, so you can use that avenue. Twitter might work, too. But don't be disappointed if they don't react; many users want to keep their Stack Exchange presence separate from their other (online and offline) activities.