Similar to the OP, myself and my daughter have separate EBay accounts, she's on Facebook, I'm not. I use Firefox, she's on Chrome. Both on Windows 7.
Earlier today I clicked on a link in the EBay Madness topic. This evening on my daughter's Facebook feed she had a photo/message from EBay saying "Inspired ? Take another look". The item being that which I had viewed.
Again yesterday I visited for the first time a site called Firebox to look at an item my daughter had suggested as a present. Within half an hour she had the item come up on her Facebook feed.
Now, earlier replies inferred that this can happen with a shared IP address, however my daughter moved from home down to London a few weeks ago, so that connection has been severed.
Thoughts anyone.
Brian