For instance, Facebook writes of its chat heads feature:
With chat heads you can keep chatting with friends even when
you’re using other apps. When friends send you messages, a chat
head appears with your friend’s face, so you see exactly who
you’re chatting with. Messages reach you no matter what you’re
doing - whether you’re checking email, browsing the web, or
listening to music.
Is that a feature or a threat?
Here’s the thing about apps, and on a larger scale, technology
that people love — no matter how much someone is into something,
they don’t want it thrust in their face 24/7.