Messages and notifications
Direct messages, who can message whom, and the notification bell.
Messages are private, one-to-one. Some memberships have daily message limits, and some member types can't start conversations with others until they upgrade — the site tells you when a limit applies.
Notifications (the bell) cover likes, comments, follows, replies and admin notices. Clicking a notification takes you straight to the post, photo or profile it refers to.
Don't want to hear from someone? Block them from their profile menu (the … button); blocked members can't message you or see your content.