Facebook's momentum can be described as any sort of a number of analogies involving velocity. I've been meaning to write a post but with so much great insight popping up, I'll leave it to aggregation and attribution.

Strategy:

• "I advise marketers not to invest too much time in creating 'a Facebookstrategy' as much as they don't have 'an NBC strategy' or 'a New YorkTimes strategy.' Instead, I encourage them to people watch, learn andthen plan based on their audience and the big picture." - Steve Rubel
• "Wal-Mart has the opportunity to build a community with these students,but has to resist the temptation of treating this group as yet anothermarketing channel." - Charlene Li
• "I want to know what's happening in the world faster than anyoneelse...If someone says something, I want to see it first." - RobertScoble via Pete Blackshaw

Tactics:

• "Join or build a community, deploy an application (widget), invest inadvertising, gather intelligence from profiles, and extend one'snetwork." - Jeremiah Owyang, "What the Web Strategist should know about Facebook."
• "Employees are more likely to view important company information when itis just one click away from their Facebook page than if it is onsomething totally different." - Shiv Singh, Avenue A/Razorfish
• The annual cost of a sponsored group is $1.2 million - Valleywag

And with all good things comes spam. The malicious-minded can create fake profiles to mine profile data.