Self-promotion may be annoying, but that doesn't mean that it's unsuccessful, it may still change others' perception of one's skill and accomplishments. It's like commercial advertising, we all hate it most of the time, but it may still work.
When I noticed by boss didn't remember I'd been volunteering, and there was no benefit for me, I stopped. The next person required to perform disaster recovery was this older guy, he bitched and moaned at every oppurtunity, making sure everyone knew. Until one day in staff meeting I'd had enough, and said something about his whining, and how I'd done so many times and no one even remembered. My boss was really taken aback. That was the beginning of the end for me at that company, I'd already stayed too long.
Note, that guy later committed suicide. He was a real miserable SOB.
Both require a careful analysis of whether a greedy behavior will be adequately penalized if you are designing a social network to not immediately become facebook.