Hey, i have a question for all SEO experts. I want to know weather social networking sites are helpful in increasing traffic or not???

I think no, social networking sites are there only to increase the traffic.

A parameters must have to use that might want to consider doing a URL rewrite to static URLs, at least for the most important pages.

Yes, social networking indeed helps you in getting loads of  traffic (provided you are participating actively). Nothing can be better than putting yourself in a place where millions of people are hanging out.
But it doesn't help much with PR. PR solely depends on number of quality backlinks pointing to your webpages.

The sites you want to be active on to increase PR are ones that allow you to use signature links ie dofollow blogs and forums (like this one). Make sure you do not spam them though - find ones you have a genuine interest in.