Twitter shares stumbled Monday morning after CEO Parag Agrawal revealed that Elon Musk decided to not join the social network's board.

The San Francisco-based company last week had announced that Musk would be joining its board of directors after the Tesla CEO paid nearly $3 billion for a 9.2% ownership stake.

But the plans fell apart on Saturday morning — the same day that Musk's two-year term was set to begin. In a tweet explaining the situation, Agrawal did not disclose the reason for Musk's about-face.

"We were excited to collaborate and clear about the risks. We also believed that having Elon as a fiduciary of the company where he, like all board members, has to act in the best interests of the company and all our shareholders was the best path forward," he wrote, adding that "I believe this is for the best."

Musk's 73,486,938 shares make him Twitter's largest shareholder. And for years, he has also been one of Twitter's most famous users, with a follower base that is now over 80 million.

In the days since his stake in Twitter was revealed, Musk has publicly called for several changes to the platform, including the addition of an edit button and the ability for users who subscribe to Twitter Blue to pay with dogecoin.

Musk's acquisition sent the social network's stock spiking more than 20% last week, but news of his decision to not join the board briefly sent the stock down as much as 8% Monday morning before it recovered.

If you had invested $1,000 in Twitter a little over five years ago on April 4, 2017, your investment would be up 213.2% as of Monday at 11:29 a.m., according to CNBC calculations, with a total market value of $3,131.72.

It would be worth less, however, if you had invested in Twitter at its 2013 IPO price of $26. The same $1,000 investment made on November 7, 2013 would be worth $1,769.42 as of Monday morning — a return of 76.9%.

Over the same two time periods, the S&P 500 index grew by 87.82% and 153.72%, respectively.

Despite Twitter's growth over the years and Musk's recent involvement, past returns of an individual stock do not predict future results. Make sure to carefully research your options before investing.

And instead of trying to predict which stocks will go up and which will go down, consider buying low-cost index funds and holding onto them. This type of diversified fund typically stays relatively constant and avoids the ups and downs that comes with picking single stocks.

This story has been updated to reflect Elon Musk's decision to not join Twitter's board and Twitter's stock price as of Monday day morning.

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