One Wall Street analyst abruptly changed his view for Facebook shares within hours after seeing a big drop in the internet giant's stock price.

Facebook announced on Thursday major changes to the company's News Feed. CEO Mark Zuckerberg said the internet firm will start prioritizing "meaningful social interactions" versus "relevant content," and that he expects the time people spend on the social network will go down as a result.

That prompted Stifel's Scott Devitt to reiterate his buy rating on the social media firm in a note to clients just after midnight Friday.

"We believe Facebook is doing the right thing for the long-term sustainability of the platform," he wrote. "We believe Facebook shares will struggle until the long-term economic implications of the proposed changes become more clear."

But he said in the note, "We remain buy-rated on FB shares but with less conviction than any previous moment of our coverage."