[netsharc]

The author's domain ends in .de so I presume he's German. Attending a data protection lecture in Germany, the lecturer said the same thing, he can technically sue his friends because they gave his name and number to a 3rd party, without his consent. So at least in his jurisdiction it's illegal, however you may disagree.

Tragically even "secure" apps like Telegram does this.

The more interesting aspect is that if a group of people, some of whom are friends, uploaded their contacts, the network (Facebook, WhatsApp, Telegram) can build a graph of friends. And they can build a profile of you based on the profiles they have from your FB-using friends. Not sure how exact it'll be, but it's what they do and sell to their customers.