I see a few big blogs make so many quilts using only designer fabrics and I'm wondering if they are given the fabrics in order to advertise the fabric line. Is this possible?

Anything is possible!
And perhaps they just happen to like a particular line of fabric?
And perhaps they have a store, and are promoting their own product lines?

Probably. I wouldn't doubt it. Others just prefer to only use designer fabrics and pay the $$$ to always have them.

Or maybe they fit into the "average" quilter demographic. According to the last survey the average quilter is affluent. We're not poor but I still have a quilt "budget" and I use "designer" fabrics. I don't buy any fabrics at Wal-Mart or JoAnn's. Just because I only use LQS fabrics doesn't mean I paid full price for them.

And "they", whomever they are, also show their Long Arm stitching as a way to advertise without putting it into 'Classified'.

If it's their blog ....."they" can do whatever they wish to do on their blog.

I too, try only buying "designer" fabric. I feel if I put that much time in something I want the assurance of good quality fabric. I do get most on sale, and watch for designers I know give good quality all the time. My first quilts had fabics that were not as nice, I definitely can tell the difference by the feel.

I get to sew on designer fabrics cause I make samples for the LQS. It taught me that there is a big difference at times in the quality. But, then you do pay for that quality. But, now that I have slowed down a bit in my quilting, I am able to pay more for my fabric as I am not going through it so fast, so I go for the designer fabrics when I can.

You could always ask them.

I have enough disposable income to buy designer fabric but I seldom pay full price. I'll wait for that line to go on sale or on clearance. I don't mind that is not the current LQS fabrics. I can always find a sale online.