Here's one. I think it would be useful to be able to ping a particular person with a question. A student might be looking for a particular tutor, or another student with whom she's worked with before. Maybe with a follow-up question, maybe with a related question in another context, maybe with an entirely new question. A tutor might be wanting to follow up with a particular student, on an assignment they worked on together, for example, to see how it went, or to see how a projected assignment is coming along.
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Currently the only way to do this (as far as I can see) is to find a thread in which the person you want to reach was involved and post something there, even if your current question has nothing to do with the original discussion. If there were some way, some open way naturally, to reach individuals, it would be really helpful. When the follow-up is related to a given thread, it makes sense to post there or course, to maintain the discussion intact. But there are times when this is not the case.
(Had to use the answer box to get the whole of that question in!)
We're working on pings as a possible feature right now, though I can't give you a specific ETA or anything at the moment :)