link velocity - Google's need for freshness

Link velocity refers to the speed at which new links to a webpage are formed. Historically, great bursts of new links to a specific page has been considered a red flag, the quickest way to identify a spammer trying to manipulate the results. With information suddenly so viral and speedy, bursts of links to content are key indicators of freshness and what is at the top of mind for searchers. Take for example, Wikipedia’s content is continually updated by its editors, giving it the appearance of constant freshness. Combined with continual linking, it gives it a live Web relevance and therefore placement. Age of content and websites are then devalued in favor of freshness, and link bursts regain value so long as link bursts appear naturally viral. How does a search engine differentiate between spam bursts and natural viral bursts?