As baltobetsy pointed out, breaking your own voluntary Lenten penance is not a mortal sin, so it is not necessary to Confess it. Because giving up Facebook is a personal Lenten devotion, it’s up to you to decide how strictly or loosely you will abide by it.
There’s also a matter of intent…if you were going through your email and read the Facebook alert message without consciously intending to use Facebook, then (in my opinion) you have not even broken your penance at all anyway.
It’s like the person who eats a ham sandwich on Friday, and realizes (after eating it) that it’s Friday and he broke the Lenten abstinence. Well, he didn’t mean to. That’s not sinful. It only would have been sinful if he had known it was Friday, knew it was Lent, knew he shouldn’t eat the ham sandwich, and did it anyway.
If you wish to abide strictly by your chosen penance, you might consider logging into Facebook (just once!) to turn off all of the email notifications and then log right back out. Then this won’t happen again.
God bless you!