To estimate the size of an
OSN, i.e., the number of users an OSN has, this paper introduces
three estimators using widely available OSN functionalities/services.
The rst estimator is a maximum likelihood estimator (MLE) based
on uniform sampling. An O(logn) algorithm is developed to solve the
estimator. In our experiments it is 70 times faster than the naive linear
probing algorithm. The second estimator is mark and recapture (MR),
which we employ to estimate the number of Twitter users behind its
public timeline service. The third estimator (RW) is based on random
walkers and is generalized to estimate other graph properties. In-depth
evaluations are conducted on six real OSNs to show the bias and variance
of these estimators. Our analysis addresses the challenges and pitfalls
when developing and implementing such estimators for OSNs