This is a crowded market: Big players such as Google and Facebook, hold over 50 percent of the market share, and other significant players, such as Microsoft, Oath, Twitter, and Amazon, are vying for the rest.

Google and Facebook provide marketers access to the online traffic they control and use sophisticated targeting algorithms to match a marketing message to the right set of online users.

To the extent that this targeting is effective, marketers will benefit, as precise targeting increases click-through rates and lead to conversions with lower acquisition costs.

But the effectiveness of this targeting also depends on the motivations of the online users as they surf different websites.

For example, a visitor at Facebook may be there to interact with his friends.

A visitor at Google may be searching for some content related to her studies.

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