Suppose you have three sets of data.
One is a set of site names mapped to number of users
hotmail,1500000 youtube,2000000 yahoo,10000000
Second is a set of usernames, ex.:
starbaby tassadar lucifer
Third is a set of username:site pairs, signifying the username from the set above exists in the set of that site's users e.g.
starbaby,hotmail tassadar,youtube starbaby,yahoo lucifer,yahoo tassadar,yahoo
How can you estimate the number of users of a novel site not in the first set based on the existence/non-existence of each of the usernames in the username set for that site?