"Now users can choose between some very good general-purpose browsers, so the opportunity has narrowed for a new entrant," he said.

As Valdes said, RockMelt isn't the first browser to focus on the likes of Facebook or Twitter. Flock has worked that side of the browser street for more than five years, to mixed success.

In that time, Flock has collected about 9 million users, said the firm's CEO, Shawn Hardin, who classified 2.5 million of those people as "active" users who have run the browser at least once in the last month.

Flock's active user base pales in comparison with other browsers. Mozilla, for example, claims that its Firefox has over 400 million active users, while earlier this year, Google said Chrome was being used by more than 70 million.

Firefox is the world's No. 2 browser, while Chrome is ranked No. 3. Both lag far behind Microsoft's Internet Explorer in usage share estimates.