1. A method comprising: (a) maintaining, by a computer system comprising one or more or computers, one or more databases stored on computer-readable media comprising: (1) first electronic data comprising one or more digitally created reference compact electronic representations for each of a plurality of reference electronic works, wherein each digitally created reference compact electronic representations comprises one or more extracted feature vectors of at least one of the plurality of reference electronic works; and (2) second electronic data associated with one or more of the reference electronic works and related to action information comprising displaying an advertisement corresponding to each of the one or more reference electronic works; (b) obtaining, by the computer system, a first digitally created compact electronic representation comprising one or more extracted feature vectors of a first electronic work; (c) identifying, by the computer system, a matching reference electronic work that matches the first electronic work by comparing the first digitally created compact electronic representation of the first electronic work with the first electronic data using an approximate nearest neighbor search, which is a sub-linear search of the first electronic data that identifies a match to the first digitally created compact electronic representation within a threshold but does not guarantee to identify the closest match to the first digitally created compact electronic representation; (d) determining, by the computer system, the action information corresponding to the matching reference electronic work based on the second electronic data; and (e) associating, by the computer system, the determined action information with the first electronic work.