Personally, when I don't receive a thank you note, the person no longer receives presents from me.
I've wanted to do this before... but DH would kill me for not bringing gifts to the people who don't send one out... so we still do :-/
I do think most of it has to do with a general laziness and entitlement meshed with technology. While I haven't gotten a wedding invitation by email (yet) I have gotten several birthday party, baby, and brial shower invites through FB. Some people say it's a budget thing-- I'm on a budget for LO's party but I still sent out invites! I just think it's so rude- come to a party where you are expected to give gifts but I won't bother spending a cent on inviting you.
The only thank you notes I have received are for weddings... nothing for anything else. Obviously my LO (turned 1 today) won't be doing them himself this year.... but you can bet once he learns to write after he gets a gift he will be sitting down and writing a thank you note. If he gets an attitude then guess what? Gift goes "back" ( not to the gift giver b/c that would be rude, but to mommy's closet til he earns it back) and a thank you note would be sent by mommy.