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"There's the possibility that the communications, the news, the information usefulness of Twitter could be all subsumed by Facebook," the lead Internet analyst at RBC Capital Markets told CNBC's "Squawk Box." "It's a remote possibility, but it's a much greater one than the flip side."
Shares of Facebook hit an all-time high Tuesday, prompting closely watched venture capitalist and Facebook board member Marc Andreessen to tweet out a criticism of a Barron's September 2012 cover story saying the social network was worth $15.
Responding to that tweet, Mahaney said, "At one point Facebook probably was worth $15 because they couldn't figure out mobile. Once they figured out mobile, which is where all their dramatic growth has been over the last three years, then the stock deserved to appreciate."
He said Facebook is the best bet on the big trend in Internet advertising this year: mobile and video marketing.
Mahaney further noted that photo-sharing app Instagram, which Facebook owns, is now running more advertisements. That platform can probably generate $2 billion in additional revenue for its parent company during the next year, he said.
Twitter announced last week it would follow Facebook's lead and give marketers the option to automatically play video ads that appear in Twitter streams, a move that has helped Facebook increase viewership. It will also charge marketers only when their videos come into full view and launch a news curation service that it says will make stories easier to find and follow.
To be sure, Twitter, whose CEO Dick Costolo is stepping down on next month, is ramping up revenue growth. Twitter earned $1.4 billion in 2014, up from about $635 million the previous year. But Wall Street has become concerned about slower-than-expected user growth.
RBC Capital Markets does not currently recommend Twitter's stock, Mahaney said.
"There's too much wood to chop at that company, both in terms of fixing the product for users and fixing the product for advertisers. There's a lot of uncertainty about it," he said.
Twitter is a company that cannot get out of its Silicon Valley ways, he added, saying Twitter should revisit its core policy of limiting messages to 140 characters. "There are some basic user changes they need to make to broaden their audience, and they haven't done it."