Look out Facebook!” the News Corp CEO wrote today in a tweet. “Hours spent participating per member dropping seriously. First really bad sign as seen by crappy MySpace years ago.” Easy to see why he’s still smarting over the “crappy” asset that he bought in 2005 for $508M and sold two years ago for $35M. But his warning also reflects the passion Facebook inspires among supporters and critics alike on the anniversary of its ill-fated initial public offering at $38 a share. The stock closed today at $26.25 — down 31.3% — and has been pretty much flat for more than five months. Bears say that Facebook can’t sustain its torrid growth as it faces potent competitors — including Google, Twitter and Tumblr —  and a shift among users from personal computers to advertising unfriendly small screened mobile phones and tablets. “Facebook is now scrambling to boost revenues through bigger ads that take over the entire screen,” BTIG’s Rich Greenfield notes today. He contrasts that to Google+, a social network that “is not out to harm the user experience through disruptive, annoying, spammy ads, they simply want the data to improve search and other products.”

But bulls say that Facebook remains the social media champ globally and will be hard to beat. “Other than Google, no other web publisher operates at this scale, providing [Facebook]with a real advantage,” Pivotal Research Group’s Brian Wieser says. Others note that Facebook’s making progress in building — and selling ads on — its mobile platform. “It accounted for 30% of total ad revenue in Q1, or roughly $374 million, up from $0 last year,” says Wedbush Securities’ Michael Pachter. Since the mobile initiatives “are only beginning to ramp, we expect positive dollar growth to continue at a reasonable clip for the foreseeable future.”

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