They can be edited, but only once, within 5 minutes of creation. And when you edit it, you can change any text, but you have to include all the same [[mentions]], [[links]], [[tags]], and other fancy things like [[spoilers]], [[embeds]], and [[polls]].
=== Following Others ===
No, it's not creepy. Well, it could be creepy. Don't be creepy about it.
=== Private Messaging ===
Many apps or clients let you send private messages to other users. You can send them to one or more users at once, and all of you will have a shared room to view the messages, that no one else can join or see.