To eliminate Facebook email notifications, select Account Settings in the drop-down menu on the upper right side of your Facebook page, and then select Notifications from the menu on the left side of the page, to bring up your notification preferences. Manually select each part of your Facebook account, and unclick each checked box to turn off the corresponding notification e-mail. Facebook's incredibly granular approach to settings means you'll have to uncheck a lot of boxes on this screen, but the arrangement does let you control the email notifications you receive (so you can find out when someone posts pictures of you, but skip all the game invites), and it should reduce the amount of time you spend checking your email.
To turn off Twitter's email notifications about new followers, retweets, and so on, first log in to your Twitter account and select Settings from the upper right drop-down menu; then select Notifications from the new menu on the left. Twitter has far fewer settings here than Facebook has in its modification options area. Simply turn off the six notification options, and you'll be ready to go on your way.
Is the bright red Google+ notification icon in the top-right corner of your GMail account distracting you? Regrettably, the notification toolbar in Google+ is impossible to kill from your Google settings, and if you're a GMail user you'll often find your eyes drawn to that bright red reminder. If you want to continue to use Google services without having to endure the Google+ notification bar, download Google Chrome and install the Hide Google+ Notification extension.
The Nuclear Option
If you've turned off all of your notifications but find that you still waste most of your day on Facebook and Twitter, it's time to accept that the problem might rest with you as much as with the sites themselves. If you can't help yourself, install a browser extension that blocks distracting sites automatically. As a Chrome user I prefer StayFocusd--a useful extension that completely blocks unwanted sites except during a limited time (the default duration is 10 minutes) each day. That way you can ensure that you don't miss out on important updates, without wasting your whole day on the Web.