Essay coming up, sorry!
There's some complacency here, I think, with people suggesting "if you don't press like, nothing will get posted to FB".
Trouble is this has not always been strictly true and there's no guarantee it always will be. The DM article linked to upthread may have been heavily embroidered (imagine my shock) but the original blog posts it links to raising issues about Facebook are, as far as I know, genuine and accepted as accurate in the tech community. I read them at the time and hadn't read the DM article before.
This is the original post and the update post is linked at the top. So from these posts it appears that a few months back Facebook WAS tracking people's activity on the internet even when they were logged out of Facebook for a time (tho they weren't doing anything with it). When called on it, they fixed it and now they don't track you when logged out.
The problem is not that they were evilly planning to hurl your info around the net. The problem as far as I know is that a simple mistake like that one combined with other functionalities on FB could potentially result in a one-off accidental link being made. So for example, you now only have to say "yes" once to most apps for them to have permission to share everything you do on certain sites. Hence lots of people's Last.fm usage is continuously scrolling past on my updates page.
So yes as I see it, it could be a problem in the future, even if it isn't now. It's your call as a user whether you want to take the risk of having a profile on both, and how much keeping up with tech news you're prepared to do to spot problems as they arise. One workaround being discussed at the time was to keep a separate browser just for using Facebook, but I'm not techy enough to know how unbridgeable that gap really is.
Told you it was an essay. 