Freelancing sites are a poor way to make a living long-term, but I know several people who established themselves using freelancing sites. They are one way of establishing happy customers, customers who go to you first for new work and who refer you to others.
Whatever works. Just realize you'll have a lot to unlearn when you get done working on those projects, like the fact that your real rate is anywhere from 1.5x to 5x higher than you've been getting.
Also not sure how advisable it is to set up a stable of recurring/referring customers on a rate established in competition with the types of freelancers that frequent those sites.
Yes, you're going to lose some of those early customers as you keep raising your rates. Most of them have some really bad experience with lowball bids that they'll stick with you quite a ways. As always, it's the referrals that matter most.
I don't think that is necessarily true. Majority of freelancing sites are geared towards overseas outsourcing and portray the value proposition to companies of adding money to their bottom line. This naturally makes it hard for US based developers to make market rate on these sites.
Shameless self-promotion: I am a co-founder of grouptalent.com which is geared towards high-end freelance developers and rates generally trend toward market.