Primary care has always paid the least. Average general practitioner salary for both NPs and MDs is less than in specialties. That doesn't mean an FNP salary needs to be excessively low (someone recently posted saying they are an NP making 60,000 a year... yikes.) However, in general, primary care will always pay less than specialty.
I'm a relatively new FNP and, while I could have worked in a specialty setting and made more, I prefer the primary care setting. I have so much variety in my work day. In one day I might put in an IUD, do a pediatric physical, do a home visit to a geriatric patient, and treat someone with bipolar (we take on a lot of psych because the wait list is over a year to see a psychiatrist here). I see the appeal of specialty care as well, and the extra money would be welcome, but I don't like any one particular area of medicine so much that I want to do it exclusively. I guess I'm the "jack of all trades, master of none" type