Creating tension, the need to read on, to find out what happens next, is a technique that social media posters can use to build engagement with their followers.
I'm not suggesting that each post should have a cliff hanger, but I am encouraging awareness of how storytelling techniques can be used in a corporate or other general social media environment. When you see news posts about what the iPhone 5 or iPad 6 may have when it comes out you are being teased by foreshadowing, a long established technique for building tension in stories.
Most people will not only look out for further posts in the story of what is to come from Apple, they are also more likely to become engaged with the sites that deliver those teasing posts.
Foreshadowing is only one technique for building tension. Dropping opposite types in the same room does it too. So does anticipation of an award or a disaster. So does a looming major change, a surprise announcement, a rescue story or a mystery. There are others too. Think about what engages you when you watch a movie.
You may only use one of these techniques because of your situation, but it is important to be aware that you can build engagement in many ways.