Slide, the maker of social networking widgets, has raised a big $50 million fourth round from Fidelity and T-Rowe Price. The round, sans-dollar amount, was first reported by Kara at All Things D; Brad Stone at Bits then reported the amount. The Silicon Valley-based company founded by PayPal’s Max Levchin has raised three unspecified rounds, with the last one coming in late 2006. The latest values the company at an impressive $550 million, a price tag that should embolden other investors and startups that piggyback on social networking services. Originally, Slide started by offering a tool for online slideshows, but it has since introduced a range of widgets that work on various social networks. The company now has 64 employees, a number it plans to add to with the latest round. The company is frequently described as a competitor to RockYou, another funded maker of popular social net widgets.

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Bits: Max Levchin, Slide’s CEO, trumpets the comparison to previous iterations of the technology cycle. “It’s impossible for social networks focused on scaling the network itself to build all the niche applications that bring people and keep people on these sites,” he said. Just as consumers bought Windows to play games, organize their taxes or create documents, application makers like Slide “add the bulk of perceived value to the consumers of these Web platforms,” Levchin said.