Also, I'm not sure GM's marketing dept. is experienced in advertisement for online media
Sure, it's easy to advertise on FB (or Google Ads). It's a whole different story to make it effective.
Take Google AdWords. If you don't optimize your strategy you'll have a high CPC (cost per click), will probably pay for ineffective words. Also, there are changes you can do to your content (being advertised) that affect your CPC.
They certainly aren't mom and pop but it doesn't mean they are up to date
They may keep an old agency just for the sake of it, who may not be specialized in modern medias
And they can hire the best people for internet marketing just to end with their campaigns not approved by GMs marketing department, having to get something conservative (hence, ineffective)
Sure, it's easy to advertise on FB (or Google Ads). It's a whole different story to make it effective.
Take Google AdWords. If you don't optimize your strategy you'll have a high CPC (cost per click), will probably pay for ineffective words. Also, there are changes you can do to your content (being advertised) that affect your CPC.