Imagine you get more mail a day than you can read. You got two choices: spend significant time filtering through or risk missing the signal for the noise.
Now imagine every message in your inbox cost someone $100. First, it would significantly cut down on the volume. Second, if you know that a stranger spent $100 to write to you, you can assume it's not completely trivial - someone must have thought that what they have to say is so valuable that you're going to care and respond that they staked money on it.
Frankly, $100 is cheap. Say I have a startup idea that I think facebook would jump all over but I have no-one in my network who can help me bring it to FB's attention. I would GLADLY pay $100 for access to Zuckerberg - if $100 is enough to bring his inbox to a reasonable size such that my genuinely good idea could get the attention it deserves, it's well worth it. Frankly I think $100 is too cheap for someone at as high a profile as Zuckerberg.
Many of us get LinkedIn email from recruiters that we generally proceed to ignore. Now let's say a recruiter had to pay $5 to email me (if they weren't in my network): it would both cut down the amount of noise, and make me likely to take the email more seriously: if the recruiter was willing to put up money to make me aware of his opportunity, maybe there's something there.
Similarly, imagine it cost $5 to send your resume to a company. It would immediately stop people submitting their resumes for every posting in the world. The company could rely on the fact that any application for any position is from someone who genuinely believes they are a match and perhaps do away with machine resume filters, if the volume was brought down enough. In other words: although it would seem "greedy" to charge people $5 to apply for your job, it would end up meaning that more of the better candidates made it further through the process.
In general, putting a $ figure on a communication significantly increases the signal to noise ratio. $100 for Zuckerberg's attention is fair. $5 for my attention on LinkedIn is probably fair too - especially if I could set my own price. If I don't get anyone contacting me, I drop the price. If I get too many bogus offers, I raise it.