Selective replacement of data maintained by third-party network services

A network computer system implementing a web service, the network computer system comprising:

• one or more processors;
• one or more memory resources storing a set of instructions, that, when executed by the one or more processors, cause the one or more processors to:
• maintain a token pool including a plurality of unique tokens, each unique token comprising a string of characters having a particular format;
• receive a first web service call from a third-party network service, the first web service call corresponding to data that includes a data item;
• identify a character string in the data item that has a format that coincides with the particular format;
• select a token from the token pool;
• provide a response to the first web service call that causes the third-party network service to store a modified data item, where the modified data item is generated by replacing the character string in the data item with the token;
• and
• replenish the token pool with one or more unique tokens having the particular format.