I have asked a question on Stack Overflow and have not got an answer yet. I know a person on the site who I reckon to be able to answer it (as they are the author of the library at hand). Unfortunately, I have got no social media of theirs and cannot share the question with them directly. Instead, I have found one of their posts on a similar matter. Is it OK to link my question in the comments? Or just comment the question itself in a concise form? Or is there some other way?
The culture of each SE site (there are almost 200 of them!) is different. In my experience most sites are pretty comfortable with an occasional well-placed short comment alerting a post author to a related question, and in some sites it's welcomed and happens all the time.
In Stack Overflow however I have received indignant scoldings for exactly the same thing that I've been thanked for in other sites.
One way to proceed would be to just use the single word "related:" then the link to your question. It doesn't say "Can you answer this?" but if they can then and they're active then they may just go ahead and do so.
By the way, an additional benefit to linking to one question in a comment on another related question or its answer is that the two questions become permanently linked! At the top of a question page on the right sidebar (on a laptop web browser at least, don't know about phones) there is a Linked section. All future readers of one page will be able to see the links to the other questions.
This will permanently increase the chances of your question getting an answer because more people will see it!
These are permanent and even if you delete your comment, the items in the Linked sidebar remain.
I would say yes!
Just remember to be polite and respectful. I see nothing wrong with something along the lines of:
Hey, @user, I posted
[this question](link)(*) about the XYZ library. I'd appreciate it if you can have a look at it whenever you have the time. Thank you!
If they have the time and are interested, they can check it out. If not, they can ignore the comment (or leave a polite reply) and move on; not a big deal. Once the comment serves its purpose, it can be flagged as "no longer needed" and deleted.
(*) As suggested in the comments, it might be better to include the id of the post instead of the link (unless you're commenting on a post that's actually related to your question).