I've been getting these too for the last few days. I THINK it only started after my Nexus 5 upgraded to Marshmallow. I did have non-trusted sources turned on for a short while as I was installing Amazon Prime Instant Video but that is off again now.
I have run a scan with Avast but it has found no infections, I have installed Ad-Blocker thinking it was being triggered by ads but it is still happening.
My feeling is we have a new attack, does anyone think uninstalling Chrome could fix it?
EDIT: For the record I don't have facebook installed so it's not that.