[cbcoutinho]

It isn't just Facebook that has been doing this. Web development tools are able to track scroll and mouse movements as well, and have been used to test website usage

[angott]

There is a big difference between using this kind of technology to run usability studies or to provide more targeted advertising.

[kaybe]

Not really. If you record and save that data, the reason is secondary.

[jhowell]

Tracking an individuals mouse movement across devices for perpetuity, packaging said data as a product offering to potential advertisers, seems to be a more extensive undertaking than simply UI/UX improvements.