Thanks for a detailed reply, gore)
Would make for a nice humourous book. I did deliberate before posting these thoughts - it's not that I came out with them out of the blue.
One small note before I go on: "media consumption" is a professional term as in a phrase "How Media Consumption Has Changed Since ...".
The facts I wrote are just those that concern the Internet side of things. She has been following me a few times in real life too, but this is a computer security forum. It's been going on for about a year: a smallest change in my profile (like adding a new favourite quote - that does not appear in friends' news) prompted questions from her - and you have to really monitor one's profile to notice such small details.
If you add a song (or merely search for it - I don't remeber) - that's not in the friends' news section too - and the next day she posts it;
If you once in 6 months you go to find a video of that Italian singer, listen to a couple of songs, one of which is random and you don't even like it - and the next day she posts exactly these two videos;
If she posts a note on the lyrics of a song you recently added (not in the friends' news section as well) and listened to just the night before;
If she tells you she knows you're dating someone, whereas you don't discuss it on the social network at all (perhaps personally with friends only, and there are no common friends);
you can start to have ideas.
N.B. She does copy my music and other predilections which are open to the public, and I have no problem with it, but I have a feeling she could have crossed the line inquiring into my likes.
Finally, I was fine and I could believe it was all coincidence until that Youtube thing happened - if you consider facts, there is a very small chance of that happening.
Thank you for advice. What I need to know though is if there are programmes that could get on my computer unbeknownst to me that might be giving her that information.