If you want a librarian to monitor a specific library-related discussion board, there are a few things to consider:
What is the point?
How much work?
Graded or not?
1) Do you want it to be a basic "Ask the librarian questions here" format, or do you want it to be a place where students post specific things (or an assignment!) for the librarian to review or comment on?
2) How much work do you want the board to be for the librarian (and how much work can the librarian deal with reasonably)? A basic "Ask questions" board doesn't need a lot of time, but needs to be monitored pretty often. A graded board about structuring citations or search phrases may not need to be monitored daily, but will require a lot more work for each post (and there will likely be a lot more posts).
3) Do you want this to be graded? If so, do you want the librarian to do the grading? Make sure all this is agreed on ahead of time so there are no surprises.
Both formats serve an important purpose, and both involve the librarian in the class in a deeper way than having a "one-shot" in-class presentation about library resources.