First, it is good Netiquette to keep your address book private. Then, don’t send invites to people in your address book. Given, it is spam.
Invite Netiquette Personal Email
So, email addresses can easily be shared. Also, they are valuable. Next, good friends are extremely valuable. It follows, many sign up forms request access to personal email address books. Then, automatically email your contacts with your permission.
In any event, there are options to skip the sign up step for sending an email to contacts. Given, the address is the contacts personal information. So, it is not polite to share personal information without permission.
Personal Email Invite
Do not share personal email addresses without permission from the contact. Messages within social networks are more discretionary. Social networks provide public profile for viewing. There are privacy settings to control contact.
Keep address books private because many social networks ask to email contacts, which may annoy them.