  We consider the online stochastic matching problem proposed by Feldman et al.
[FMMM09] as a model of display ad allocation. We are given a bipartite graph;
one side of the graph corresponds to a fixed set of bins and the other side
represents the set of possible ball types. At each time step, a ball is sampled
independently from the given distribution and it needs to be matched upon its
arrival to an empty bin. The goal is to maximize the number of allocations.
  We present an online algorithm for this problem with a competitive ratio of
0.702. Before our result, algorithms with a competitive ratio better than
$1-1/e$ were known under the assumption that the expected number of arriving
balls of each type is integral. A key idea of the algorithm is to collect
statistics about the decisions of the optimum offline solution using Monte
Carlo sampling and use those statistics to guide the decisions of the online
algorithm. We also show that our algorithm achieves a competitive ratio of
0.705 when the rates are integral.
  On the hardness side, we prove that no online algorithm can have a
competitive ratio better than 0.823 under the known distribution model (and
henceforth under the permutation model). This improves upon the 5/6 hardness
result proved by Goel and Mehta \cite{GM08} for the permutation model.
