A method for online advertising includes: receiving a request for a content stream from a client device; embedding an advertisement in the content stream, and transmitting the content stream to the client device; determining a scroll dwell time of the advertisement when the content stream is presented on the client device; determining a level of performance of a guaranteed presentation amount associated with the advertisement, the level of performance based on the scroll dwell time of the advertisement. In another embodiment, candidate advertisements are identified for presentation in the content stream; for each candidate, an expected cost per time unit (eCPTU) of exposure is determined; one of the candidate advertisements is selected for inclusion in the content stream based on the eCPTU's; the content stream is transmitted to the client device.