Based on the recent surge and frequency of GPN posts on Facebook, together with the new pictures, and the fact that all posts are always right at the top of my news feed all the time...My guess is the "marketing team that manages GPNs Facebook team" that you reference above does indeed exist and hey are indeed paying the sponsor fees.  whether the band is paying it, the record company, or this marketing company I couldn't say- but someone must be paying the sponsored page fee.
If I am wrong it is quite amazing the number and frequency of GPN updates I am getting on Facebook.  To think that they are doing all that in effort to just reach 10 percent of their fan base to me seems a little crazy.  Clearly there has been a change recently in GPNs status updates...to me it is a sign of the new Facebook monster and how they chose to adapt to it.  But that is just my guess.

I think there is a much bigger picture though, regarding the evolution of Facebook itself, the overload of information and the time many of us spend on the website, and how we spend that time.  There is also the angle of how much does someone like something just for clicking "like" .  How many GPN "likes" are just Chesney fans or someone that wathes one tree hill?People's Facebook attention span is down to about half a second right now.  GPN is a live band first and foremost so if 90 percent of their Facebook updates say "buy presale Tix" or "buy this tour poster" I am really ok with that.  Can't speak for others but I don't even have time anymore to click on all te links and videos of everything I "like" on Facebook.  It has just become too much.  It used to be fun but now it's like an overload.  GPN is getting the info out there to bring the fans to the live shows and get them off the computer.   They are promoting the live shows as they should.  Just my opinion.  Sorry for typos I'm on my phone