Just how long should that Tweet or video be? This infographic from Socially Stacked gives a visual of the data gathered by Buffer.
Alice Keeler, a teacher who shares her instructional videos about Google Apps for Education, recommends keeping videos under a minute. If you can't fit it all in, break your instruction into parts, and string them together in your YouTube channel.
Discuss TLDR (Too Long; Didn't Read) with your students and ask them for examples, and look at all those emails and links you've saved to read when you have time. Then try to condense your message on social media, too!