Upvoted because this is a great debate!
I think the circumstances are different in this case. Many people I know don't like Facebook or the idea of Facebook anymore, so Google may have a hard time persuading people Social Networks are still the new-new thing. In the case of Netscape, browser use was growing like crazy and still is 17 years later.
In a way Netscape helped tear it's own head off. There was the Netscape 6 nail in the coffin. The team rewrote the entire product from scratch. It's been oft quoted as an example of why not to do a rewrite. Facebook's engineering team appears to be strong on the surface.
There is no network effect with browsers and social networks have the strongest network effect of them all. So Google has to overcome that too.
Google does use second-mover advantage to great effect though, as MS has. Imitating is fun when you're a monopoly with a great engineering team. Companies they've taken ideas from:
Symbian
Nokia
Hotmail
Apple's App Store
iOS
MapQuest
Yahoo Finance
Altavista and all other search engines.
GoTo.com (first PPC ads)