Rav Moshe zt”l maintained (Igros Moshe, O.C. 5:28:5 and Addendum to O.C. 4:89) that an eruv of tzuras hapesachim was not permissible in Boro Park and Flatbush since he was led to believe that each neighborhood individually contained a population in excess of shishim ribo (see Does the Eruv Encompass Shishim Ribo?).
These are the facts:
Census figures reveal that in Boro Park the eruv includes a population of less than 100,000 people and in Flatbush a population under 200,000 people (NYC Department of City Planning, Community District Profiles, 2006). Had Rav Moshe known these figures, he certainly would have agreed to an eruv of tzuras hapesachim in these Brooklyn communities, which would set them off from their borough, just as he allowed that an eruv in Kew Gardens Hills, Queens and the eruvin in European communities set them off from their respective cities (Igros Moshe, O.C. 4:86, 5:28:5; see also Brooklyn and Queens: Same or Different?).