Western blotting is used to detect proteins and analyze their expression using antibodies, but it does have its limitations. Flow cytometry also employs antibodies, but it uses fluorescence to detect and analyze protein expression on a cell-by-cell basis.
This video describes four situations where it can be worth going with flow over western blot: 1) Analysis of multiple targets (multiplexing), 2) analysis of cell subpopulations 3), intracellular signaling events, 4) correlative analysis.