I feel like this should go without saying, because it really should be obvious, but if a recruiter, networker, eager college student, former co-worker, or whatever contact you with an opportunity, or to ask a favor, you should respond to them.
I'm not saying you have to do whatever they ask. I'm just saying you should shoot something back.
If a recruiter contacts you wanting you to apply for a job, you certainly don't have to apply. But unless you're getting dozens of contacts a day (and perhaps even if you are), you should write them back anyway, thanking them for the offer, and telling them what you do want (whether that's other kinds of job opportunities, or just to be left alone for now, or to be left alone forever).
If a networker wants a favor, again, you totally don't have to do it. But you should write them to let them know (and if you have the time or inclination, let them know why).
Like I said before, this is something that I think should go without saying: sticking with a basic level of business etiquette not only preserves your reputation, but keeps doors open for peeking your head through in the future. You don't know what 'future you' will want. And that eager but annoying college student could be the next
Or they could just not realize that they're being annoying, and you've lost a great opportunity for a one-email mentorship: showing them that professionals respond to people, and maybe telling them what they're doing wrong.
But I'm writing this because this is not happening now, at least as far as I can tell.
I coordinate hiring for my workplace, and I've reached out to a decent number of people about available positions and internships. This includes people I know, and people I don't know. The vast majority are more or less entry-level; either current college students, recent grads, or people with just a few years experience.
And most of them (yes, most of them) never respond. Not even a "Thanks for thinking of me!" Not even people I know personally, not even on messages sent via Facebook where it tells you that the other person saw the message.
This boggles my mind. Is this normal? Am I the weird one? Is it just that we young people consider it OK to ignore any type of communication, since we can freely screen calls and ignore IM's in ways our parents never could?
It can't be. Because that would be silly.
If it's a sales pitch, sure, it's standard to dump those (especially ones that can reasonably be considered spam). Or if it's a recruiter that you've interacted with before to negative effect (though I'd still argue it's better to send them a "I'm not interested, please don't contact me again" at that point), fine. But a real message from a real person?
What do you think? Am I a total nutter to want a "Thanks, but I'm set!" instead of radio silence?