I agree with hynek that the use of the informal "Du" is widespread
today. In spite of this (or, rather, all the more so) I consider it
rude. It is obviously directed at creating a personal link which is
then exploited for selling stuff. In my view, this is precisely what
Pinboard is not, and should not appear to be. But if the informal "Du"
is used, please use it with a capital letter ("Du" instead of "du") so
that it conveys at least some respect.
David