As toufiqbarhamov points out, your privacy is being compromised by other people all the time on the social networks you avoid. Facebook, e.g., keeps a shadow profile on you.
When I finally caved and joined (something I hope to undo shortly) I was immediately informed of all the people I know who are already on the network, not from the contacts I explicitly didn’t share with them, but from the people who searched for me before I made an account. I’ve never given FB my phone number, but I’m sure they have it in a database somewhere. They probably knew my face before I ever signed up.
When I finally caved and joined (something I hope to undo shortly) I was immediately informed of all the people I know who are already on the network, not from the contacts I explicitly didn’t share with them, but from the people who searched for me before I made an account. I’ve never given FB my phone number, but I’m sure they have it in a database somewhere. They probably knew my face before I ever signed up.