Blogboing

good post from seomoz

some highlights:

stumble upon

Formula:

(Initial stumbler audience / # domain) + ((% stumbler audience / # domain) + organic bonus – nonfriend) – (% stumbler audience + organic bonus) + N

Description:

The initial stumbler “power” (Audience of the initial stumbler divided by the amount of times that stumbler has stumbled the given domain) is added to the sum of all the subsequent stumbler’s powers.

Subsequent stumbler power is ((Percentage of audience stumbler makes up divided by the number of times given stumbler has stumbled domain) + a predetermined power boost for using the toolbar - a predetermined power drain if stumblers are connected)  + (% of the stumbler audience + a predetermined boost for using the toolbar)

N is a “safety variable” so that the assumed algorithm is flexible. It represents a random number.

Del.icio.us

Formula:

Points = (Amount of times story has been bookmarked in the last 3600 seconds)

Description:

Rank on Del.icio.us Popular is determined by comparing points. Points represent the amount of times a story has been bookmarked in the last hour. The higher the rate, the higher the points. Every bookmark counts as one point.
3600 is the seconds in one hour.

good investigating post i am not sure these are the only parameters nor what these sites use against “popularity fraud” but still interesting reading.