Mozilla, which develops Firefox, was more cautious. "Over the last decade, we've seen many browsers come and go," a Mozilla spokeswoman said. "RockMelt was developed in secret so we haven't had a chance to fully evaluate it, but we'll be interested to see if and how it addresses people's privacy concerns and if it finds a place in the market."

Microsoft declined to comment on RockMelt's debut.

"Any new browser has to offer something really, really compelling," Hilwa said. "RockMelt and Flock have latched onto a large user base, but the question is whether a significant portion of the 500 million Facebook users will think something like that is worth their while."

Or maybe RockMelt is simply hoping to get bought by Facebook, Valdes speculated. "The social Web is hot right now, [and] a social browser serves the purpose of grabbing some attention and interest from early adopters," Valdes said. "If RockMelt can capitalize on that interest with other innovations, they might get enough traction to get acquired by Facebook."

Users can at the company's Web site. The , meanwhile, can be downloaded from Flock's site.