Expanding on this... Don't use mindless browsing as a filler at all. Ever.
If your build is gonna take a minute, don't reflexively switch over to Facebook/email. Sit there and think/meditate. Stand up or do some pushups.
Browsing Facebook is fine when that's the activity you're doing. Set aside some Facebook browsing time, timebox it, and stick to that.
"multitasking" is the devil. Don't do it. Better to be blank and let your mind rest for a moment instead of trying to timeslice something in. Focus on each thing separately and you'll end up more productive and efficient and also feel more personally gratified after accomplishing specific tasks one by one vs. a mishmash of "multitasking" where you're not really sure what you even did at the end of the day.