The number of transactions included in the blockchain each day
Daily Transaction count is one of the most important and controversial metrics for the bitcoin network. Because all confirmed transactions pay a fee, each confirmed transaction represents someone's desire to send a bitcoin transaction instead of any alternative use of that cost.
Bitcoin transactions can be executed and automated by software, but this is limited by the bandwidth of the network and required fees. Occasionally large numbers of transactions will be made in a short time interval, leading to long confirmation times and some transactions that may not confirm at all. While some attribute unusually high transaction volume to "spam" transactions, others hold that any transactions following the rules of the network are valid.
Because transactions have a real world cost, daily transaction count is one of the best ways to model growth in users of the bitcoin network. Daily transaction count could be manipulated in the short term, but it's difficult to identify plausible motives given the high costs.