I don't know, it just depends. If I'm in a relationship with someone, I have no problem with them checking my Facebook, emails, texts, etc. occasionally. But the way I see it is if they want to check mine, then I should be able to check theirs, too. I'm not the kind of person that sits around, thinking of ways to figure out if I'm being cheated on, but if I'm being questioned, then they should be too. I do think it's a little much when every time you hang out with a boyfriend/girlfriend, they want to read all of your **** to make sure you aren't talking to someone else. But if you have nothing to hide, it should be no big deal.
As far as friends, family, etc. go, I would never let them read texts or anything unless I'm showing them a specific one. If they do go through my phone or emails, that's just rude and I will slap a bitch.
If you're dating someone who has your Facebook wall, email, etc. saved in their favorites, you need to move on because that stalking **** isn't cute.