Rather than just give the single biggest mistake, I’ll give a four things that drive me bonkers.
The first is not taking it seriously or thinking “it’s just a resume.”
If Linkedin is “just a resume” than the internet is “just digital plumbing.”
Linkedin Profiles support rich media content, and can tell a dynamic and interactive story from a variety of perspectives. So no, it’s not just a resume.
The second thing is ignoring the importance of keywords and SEO for Linkedin. Here’s a pro-tip, type in what you want to be known for in the Linkedin search box, and study the top profile. There are reasons why that person ranks highly. Think about how you want to be found on Linkedin.
The third thing is a bad profile photo and/or cover photo. The photo is the first thing people look at, get your sunburned vacation selfie off of the world’s most use professional social network.
Finally, not using rich media! Linkedin added the ability to embed videos, pictures, slides, documents and more. Why wouldn’t everyone use this? Who wants to read a bunch of text?
Use rich media embeds to tell a more interesting story than you could with words alone.