If you’re needing an Invite to Pinterest, you don’t have to get one from your friends on Facebook or Pinterest – you can request one from the Pinterest Homepage.
First click the “Request an Invite” button.
Then fill in your email address (same as the one you used for Facebook is convenient) and click the “Request Invitation” button.
Within a few days you will have to click some links in emails and fill out some forms. When you do, connect your account to Facebook to pre-fill your list of people you Follow, and make yourself available to them for Following.