[kaolinite]

You'll need to do research into this yourself but I believe that most marketing and sales companies uses CRMs, e.g. Salesforce, to manage their contacts. As you've mentioned, Android/Gmail syncs contacts and I believe that iCloud does too?

As for "every Tom, Dick and Harry" being added to your contacts list, this is a good thing, though I agree it leads to a messy contacts list. The contacts search on Gmail (via the "To" field on a new email) is so good that it doesn't really matter if you have a huge list of contacts - it will single out the contacts that you email the most and put them top of the list.