Currently there seems to be no functionality available to share a link to an unanswered or already answered question with another Math.SE user.
Indeed, if the question has no answers, then scrolling further down one can find the text "Know someone who can answer? Share a link to this question via email, Google+, Twitter, or Facebook" with hyperlinks triggering the corresponding action. If the question is answered, this suggestion is not displayed at all. Well, one can still share the question clicking on the
share link just below the question itself, without a need to scroll further down (That link has no "email" option, though it allows to generate an example citation). Anyhow, what may be missing in both is an ability to specify a username of another Math.SE user.
Let's consider what happens as a result. When User X sees question and knows that User Y may be interested to have a look at it and maybe will even be able to provide a useful answer, there seem to be no straightforward way for User X to notify User Y via Math.SE. Instead, User X may use the following workaround, which takes more than two clicks, though, but it works. One need just to find some discussion where User Y has participated earlier, and post a comment with a hyperlink there, so that it will appear in the inbox of user Y. This works and may be regarded as useful by both User Y who will discover the question earlier, and by the OP who may get faster help.
This, however, populates the history of comments with comments which are mostly irrelevant to the original question. Such comments may be distracting for the reader who is looking for an answer to the original question. Cleaning them up will create an extra administrative overhead to moderators/administrators of Math.SE. I also presume that posted links may trick the routines which selects linked and related questions to be displayed to the right of the discussion.
I did not manage to find previous post on this matter, so may I suggest a discussion on whether it may be sensible to add functionality for sending a direct notification about a question to another Math.SE user? Would others find this useful? Which measures should be taken to prevent excessive use of this feature (e.g. a privilege requiring certain reputation, or a limit permitting posting no more than N notifications a day or, say, posting the next notification only after next T minutes, etc.)?