I'm getting really annoyed at the number of apps which assume they should be allowed to look at your contacts list. I've got an account with no actual contacts and minimal information (just my initial and surname, so I get to be quite common in phone books etc) which I use for Google+, Disqus, etc,
I did try out Twitter 3+ years ago and even registered with LinkedIn (which found some people I'd really not have expected, such as a senior lecturer who left the place (we worked at) in 1985.
A school friend living in Denmark did get in touch having found me from a search which came across a page on someone else's website, but luckily I've had very few unexpected contacts from people I'm happy to have lost touch with (!)
I'd recommend using SpamGourmet.com which allows you to create unique e-mail address for every web site you use (and forwards to your real e-mail account). If the website you use is hacked, or sells mail addresses, you can let SpamGourmet nosh on the mail messages. It will, by default, after 20 items, unless you act to 'whitelist' wanted senders, but you can set a low limit and it will gobble thousands of mail messages without you ever being interrupted by them.
Here are stats the website about one of my several SpamGourmet user accounts:
Your message stats: 24,037 forwarded, 31,585 eaten.
You have 348 disposable address(es).
NB I have 300,000 items of unread mail right now, sitting in at least half a dozen Google Mail accounts, this spamgourmet account forwards to just one of the Google accounts.
I would estimate that at least 100,000 junk messages didn't get to me and were swallowed by SpamGourmet. Some of them will have been sites I just stopped using, others may have been hacked, or sold my 'unique' address after I stopped being a customer/ user.