I get the message "This site can't be reached" pretty much all the time. It makes some suggestions, but it's not been helpful.

This site can’t be reached

www.facebook.com unexpectedly closed the connection.
Try:
Reloading the page DIDN'T WORK
Checking the connection WORKS FINE
Checking the proxy and the firewall I DON'T KNOW WHETHER THIS IS SHARED WITH OTHER BROWSERS, CHROME AND SAFARI WORK FINE.
Check your internet connection. WORKS FIE.
Check any cables and reboot any routers, modems, or other network devices you may be using. WORKS FINE.
Allow Opera to access the network in your firewall or antivirus settings. If it is already listed as a program allowed to access the network, try removing it from the list and adding it again. I USE AVAST BUT I DON'T KNOW WHAT THIS MEANS.
If you use a proxy server... I DONT THINK I DO THIS.

So...

Other browsers in my macbook pro such as chrome work fine.
Other devices in the same network work fine.

I've found that if I delete browsing history data in Opera, it starts working again and I can go into gmail and facebook..., but after a few hours it stops working and I get the error message again.