Man of Honour
That's not it. It's simply a Facebook privacy setting buried deep in the profile settings called "Off Facebook Activity". It logs all the webpages you visit whilst you have a Facebook tab open.
I think it's worth noting that the setting on Facebook does not say these things:
1) That it only logs which webpages you visit.
2) That it only collects data about you from other sites while you have a Facebook tab open.
3) That changing that privacy setting in Facebook means that it stops collecting data about you from other sites.
I know that (1) is wrong. It collects far more data than that. Facebook is quite open about how much data the code on other pages collects - it's part of their advertising to businesses.
It would be silly for (2) to be right. Why would Facebook put so much effort into data-gathering and then ignore most of their opportunities to do so?
I see no reason why (3) would be right. The whole point of Facebook is to gather data - that's what its business is.