Last week, LinkedIn celebrated its 13th birthday. It's officially a teenager. With more than 414 million users, it's the largest professional social network. And it's getting bigger by the day, making it the biggest pool of talent online as well. When it comes to LinkedIn, we're all little fishes in a big pond. Which means, if you want to be contacted about great business and career opportunities through LinkedIn, you need to stand out. However, that's where many users are making a critical mistake.

99 percent of users fail to optimize their profile.

Most of LinkedIn's members don't know how to optimize the content of their profile so they can be found. It all comes down to putting in A) the right words, and B) the right amount of words. Here's why.

No keywords = no visibility.

LinkedIn is a search tool. Just like Google, if you want to find what you are looking for, you need to put in specific keywords to narrow down the search results. For example, if I'm looking for a marketing professional with social media experience, I would put in "marketing" and "social media." The LinkedIn platform then searches through all the candidates with those skill sets and looks at how many times those keywords show up throughout their profile. This is known as "keyword density." The more density you have for the search term, the higher up in the search results your profile goes. (This quick video explains it all.)

Your headline is prime real estate.

In addition to keyword density, your headline is critical to being found. If you have the keyword in your headline, LinkedIn sees you as an even greater match and pushes you further up the search results. Thus, placement of keywords matters as much as density.

And don't forget those endorsements.

Many people have dismissed the Skills & Endorsements section of their LinkedIn profiles, assuming they don't really mean much. Unfortunately, they're wrong. The number of endorsements you get for a skill set counts toward your keyword density for that term. Therefore, the more endorsements you have, the more likely you'll show up in the search results of someone looking for that skill set.

If you build it, they will come.

Spending time optimizing your LinkedIn profile is worth it. The more active you are on LinkedIn, the better your results will be. Remember, the number of LinkedIn users is only going to grow, creating more competition in search results. The longer you put off optimizing your profile, the lower you'll show on the list--if you show up at all.