David Einhorn's Greenlight Capital bought Twitter shares, according to an investor letter sent to clients on Tuesday.

"We initiated a small position in Twitter at an average [price] of $21.59," the letter said. "Despite a massive user base and broad reach, Twitter has an enterprise value of about 2% of Facebook, the largest social media platform."

The investor also said the fund started long positions in Brighthouse Financial, Ensco and Time Warner.

Einhorn explained Time Warner will likely get acquired by AT&T, but the company's stock will do well even if the deal doesn't happen.

"We think that the Department of Justice has a weak anti-trust case and the merger is likely to go through despite being challenged in court," he wrote. "We also believe TWX has several strategic options to create value if the deal with AT&T falls apart."

Facebook announced on Thursday a major change to its newsfeed. CEO Mark Zuckerberg said the internet giant will start prioritizing "meaningful social interactions" versus "relevant content."

Twitter shares rose 4.4 percent Friday, while Facebook stock declined 4.5 percent the day after the announcement.

— CNBC's Scott Wapner contributed to this story.