How do I send a message to a particular user for clarification of a specific post?
The short answer is You can't. But:
If you whant to add attention on the specific post, write comment with @username text inside, and user will get a notification about your comment.
If user place links to Twitter, Github, and their personal website, you can check out these links, and often can see email addresses. Also, many users place their emails and Linked In profile links in their publicly available "About Me" section, especially if they are interested in networking
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