Very obscure job posting for faculty position. Pass onward, or keep to myself? I am a contract-limited faculty member in the United States. My subfield is small, but known for its warmth and intimacy.

A friend of mine in a different country recently sent me a job posting advertised by another institution in the United States. I have absolutely no idea where he found it. I didn't even know that this school had a department in my field, frankly. The deadline is in three weeks, but the job posting has not made its way to the central repository of job ads in my field, nor hit any of the major mailing lists. Yet it's a coveted tenure-track position that seems legitimate, described in a Word document hosted on the webpage of the department in question (though not directly linked to from it, as far as I can tell). I've never seen a job more poorly publicized. I suspect that this department is hiring someone in my subfield for the first time ever, so I can understand not knowing about the appropriate mailing lists, but beyond that it seems awfully odd.

Anyway, my question is about ethics and competition. The first thing I wanted to do was to send the job posting around to a circle of colleagues who, like me, are looking for long-term academic employment. Of course that brings with it the risk that the job could be given to someone who wouldn't have heard of it otherwise, but I just can't get comfortable with the idea of sneakily keeping it to myself. Not when the support of my colleagues is one of the major reasons I was able to get to this level. I'll probably apply for the job, of course; but if I don't get it, I want people I like to be in the running!

Or is that utterly foolish and naive?

I guess I'm trying to figure out where the line is between being helpful and being so generous that I'm shooting myself in the foot. In this case, I can't find it by intuition alone. Would appreciate input. Thank you!
You are not obligated (unless by your own moral principles) to facilitate the distribution of job ads. 

You could take a broad self-interested perspective. In this case, you decide whether and how much you want the job and get a sense of your chance of getting the job without further distributing the job ad. Then, you think about how much you value others you care about getting the job and how much you value the profession as a whole making a better selection decision, and how much more likely that is to occur if you were to distribute the ad either to colleagues or to broader mailing lists, and also a sense of the cost in terms of your chances of not getting the job. From there, you can see what most aligns with your values and goals.

In general, I would think of job applications as a domain of life that is often governed by the principles of fair competition. By fair competition, I mean the idea that it's okay to focus on yourself. You shouldn't go out of your way to hurt other applicants, but that equally there is no expectation that you will help other applicants also (especially when you are competing for a single prize). Thus, if you learn information that might help your application (e.g., interests of the employer, what they're looking for, etc.), you are under no obligation to share this with other applicants. If you know about the job, and others don't, you are under no obligation to share it. The assumption here is that it is a domain of life where the ethics of competition operate. The assumption is that other "competitors" in some sense are also operating by these rules.

Anyway, these are two perspectives. My sense is that if you're very keen on getting the job and you think that you have a reasonable shot at getting it, that it would be reasonable to keep it to yourself.