Methods, apparatus, systems and articles of manufacture are disclosed to generate measures of unique audiences for Internet-based media. An example apparatus disclosed herein includes at least one memory, instructions, and at least one processor. The processor to execute the instructions to: receive first HyperLogLog (HLL) data from a first server of a first database proprietor and second HyperLogLog (HLL) data from a second server of a second database proprietor, the first HLL data including obfuscated first user impression data and the second HLL data including obfuscated second user impression data; generate union HLL data based on the first HLL data from the first database proprietor and the second HLL data from the second database proprietor by performing a union of data sets of the obfuscated first user impression data represented in the first HLL data and the obfuscated second user impression data represented in the second HLL data; and determine a total number of deduplicated unique audience members based on the union HLL data generated by the vector analyzer.