That surely cannot hurt, but the return on investment of posting comments on a site is minute. I mean, I am addicted to commenting on HN and other sites daily and it is a lot of fun, but the notion that this is actually a good use of time for my startup is wishful thinking.
I would never expect to get a single customer from this. I think getting an article on TechCrunch is of debatable value... but the traffic you'd get from a comment underneath an article from people checking out your profile link is miniscule.
Regarding telling your friends -- again, it doesn't hurt, but it is thinking really small and is not sustainable. That might be a good strategy for winning a minor student council election, but the ROI is going to be utterly negligible. Basically, you could spend a week phoning everyone you know and convince them to check out your site. If you're lucky, this would roughly equal the amount of traffic you would get by having a moderately influential person spend 30 seconds writing a tweet about you.