Dd is 11 years old. I think most of these questions pertain to privacy for teens so I am not quite there yet.
Have you ever read their diary? No paper diary that I know of. I don't think I would look at one unless dd was acting in a manner that made me worried for her safety and wouldn't talk to anyone.
Listened in on a phone call? Well, I've been in the room when she is on the phone but she knew it. I've never secretly picked up another receiver and listened in.
Asked their friends questions about them they wouldn't answer? No. Not unless she was acting super weird and wouldn't talk and I was afraid for her safety.
Would you follow them? I can't imagine doing this.
Have you gone through their personal things, like a purse, back pack, wallet? Not really. I have looked for things with dd's permission and knowledge or checked if something is packed for a trip. I do sometimes help with room cleaning or put away clothes without much warning.
Have you looked through their room? Not really. I go in there to speak to her or put something away but I don't go in just to look through things.
How much privacy do you give your kids and how often do you feel you invade their privacy?
I give dd more privacy than she gives me. I guess I invade dd's privacy a bit but I don't think it is excessive or for the purpose of snooping.
The bathroom door has a lock which dd can use. Dd's bedroom has a door that she frequently closes. If doors are closed we are supposed to knock in our house. Dd has her own computer (no internet) and I don't look at anything on it unless she shows me. Dd doesn't have unsupervised internet access or a phone of her own. I haven't forbidden dd to use the phone but she never calls anyone. She doesn't have a Facebook or e-mail account of her own yet. I do allow her to send e-mail to certain people with my e-mail account and I do not read the messages first. I do not follow her around 24/7 to see what she is doing. She has been off in another part of the house for over an hour and I haven't checked on her.