I just read an article about increasing engagement in a 50-minute face-to-face presentation by simply allowing the audience to "reset" every 10 minutes or so by moving, changing something in the room, getting them to talk, etc. An advantage to online learning is that you don't have to fill up a certain period of time; you can easily give your learners a chance to "reset" by chunking your content into short periods of time. They automatically get a chance to reset when your video ends and they decide what to do next.
Here are some additional advantages of short videos:
It takes less time to publish/produce/render so you don't have to sit there and wait for it to go into YouTube or wherever you are putting it. I hate the feeling of waiting 10 minutes for something to produce and wondering if it should take that long or if there's something wrong and I should force it to quit and try again. Also, YouTube has a 15 minute limit; you can increase it but use the power wisely!
It gives you time to refresh in between segments, to avoid getting fatigued as you are creating the content. Recording can be exhausting, since you don't have the excitement factor of being in front of a group to keep you going (assuming you are creating the content specifically for online rather than recording a live lecture, which I usually recommend). Sit up straight and smile as you are recording - it makes a difference. If you're powering through the video just to get it done, your audience will be able to tell.
Recently I've been seeing studies about how taking breaks or microbreaks (just standing up or looking at something else) can increase productivity at work. Here's one synopsis. It's likely that your students will have Facebook open in another tab anyway - they can either 1) check it during your long video when their attention is fading and miss what you're talking about because multi-tasking is not really possible or 2) wait until after your video if you keep it short and, hopefully, engaging.
A way to keep your audience/students engaged is to tell a story. Our brains are hardwired to pay attention to stories. The professors I remember best were the ones who told lots of stories - relevant stories about their practice or clients they worked with. You can probably think of a book you read that started off with a story to hook you in. Take advantage of storytelling to help students learn.
If you have other tips or rationale for short videos, please share.
Article about increasing engagement