According to Forrester's most recent forecasts, spending on online display advertising and social media advertising in the US will grow by nearly 70% between 2017 and 2021, largely due to the rapid expansion of social media advertising. B2C marketers should read this report to understand the key changes in online display and social media advertising spending in the next five years, the dynamics driving its growth, and what the landscape will look like in 2021. This is an update of a previously published report. Forrester reviews and updates it periodically for continued relevance and accuracy; we substantially revised this edition to factor in new forecast data.