With $5 million, the investors/founders believe it can be at least a $10 million dollar company in the next 5-7 years. I personally wouldn't have made that bet but at least they are charging money for their app.
The $5 million investment did not buy the entire company, so the valuation might be north of $10 million right now. Did you perhaps mean $10 billion?
Granted VC David Hornik in the following video is referring to revenue potential, I still think that what he says implies that VC's are looking for investments with potential exits far greater than $100 million.
http://www.youtube.com/watch?v=epdTQf0ZhPE
"Billion dollar companies? We like those, but we really prefer $100 billion companies." -David Hornik, August Capital
Yammer is Founders Fund backed. 3 of the 4 partners from the Founders Fund are PayPal cofounders (Peter Thiel is the big name, but there's also Ken Howery and Luke Nosek) - Yammer's CEO David Sacks was the COO at PayPal before its acquisition.