Hi Joanna,
It's just a matter of preference. No reason you can't do both if you like; no reason not to concentrate on just one if that works better for you.
I think the reason you see so much more about Facebook in the forums is that it's quite trendy right now, and it's something that is used for many purposes by many people of all ages, so it gets exposure on multiple fronts. LinkedIn is much more focused on professional networking only, so it doesn't have that same breadth of exposure, but it still is very useful and not going away any time soon.
My personal preference is LinkedIn, but that's all it is--a personal opinion. You may find you like Facebook better yourself. You have nothing to lose by trying both--each is free, each is somewhat customizable, each can be targeted to the audience of your choice, and each can be deleted later if you should decide you don't want to bother.