Cost per mille, or CPM, is the cost of one thousand ad impressions.
CPM is commonly used in awareness and reach campaigns.
A lower CPM can mean cheaper distribution, but teams still need to check whether the impressions are reaching the right audience.
CPM can rise or fall because of audience competition, placement, creative quality, seasonality, objective, and how narrow the targeting is.
CPMs commonly range from a few dollars to over twenty depending on platform, audience, and season. Rather than chasing a number, watch whether your CPM buys impressions from the right people.
Usual causes: more advertisers competing for the same audience (especially in Q4), narrower targeting, creative fatigue lowering engagement, or a change in campaign objective or placement.
When the goal is awareness and you want maximum qualified exposure per dollar. For traffic or conversion goals, CPC and cost per conversion are the better steering metrics.