This is a case of necessity being the mother of invention. I wanted to post a comment on my Facebook wall about an episode of Big Bang Theory from season 8. Even though that was last season (we are way into season 9 at the time of this writing), I didn't want to spoil it for anyone who has not yet seen that season. And my comment did contain big spoilers.
(I have been guilty of spoilers before in real conversation without really meaning to, so trying to reform here, people!)
I couldn't find a solution on the internet that would work from the writer's side. If you are a FB user there are apps etc. that you can use to avoid spoilers about TV shows and movies but nothing that helps you write it in such a way that it doesn't spoil things for the people who read your wall. So, here's my trick.
Follow these 7 simple steps:
1. In your post, type the line "SPOILER ALERT" followed by the TV show name, season and episode that the spoiler is about, or the movie name.
2. Press enter to go to next line.
3. Then hold down ALT key on your keyboard and type 255 on the numeric keypad. This will type an invisible character. It's called a hard-space.
4. Press Enter. This creates a line that contains only one character, the hard-space.
5. Copy this line using the keyboard or mouse. Keep in mind that the hard-space will look like a normal space when you select it, but it behaves differently.
6. Paste this line about 10-12 times after your spoiler alert line.
7. Below this type your actual comment which contains the spoilers.
Now, when you post your comment, the readers will only see the Spoiler Alert line, followed by a link "See more" Now they have been warned. If they want to read your comment they can click on See more and read it, or they can just keep scrolling and not read the part of your post containing the spoilers.
If you simply typed a lot of spaces or pressed Enter to insert empty lines, that does not work because Facebook collapses all empty spaces and the post is displayed in continuation. But hard-space is treated like a character not like a space therefore, it works to create the extra space that forces Facebook to treat the post as a long post and display only the top part of it with the "see more" link below it.
Enjoy, and let others enjoy! :)