Online social network advertising platforms, such as that provided by LinkedIn, generally allow marketers to specify targeting options so that their ads appear to a desired de- mographic. Audience Expansion is a technique developed at LinkedIn to simplify targeting and identify new audi- ences with similar attributes to the original target audi- ence. We developed two methods to achieve Audience Ex- pansion: campaign-agnostic expansion and campaign-aware expansion. In this paper, we describe the details of these methods, present in-depth analysis of their trade-offs, and demonstrate a hybrid strategy that possesses the combined strength of both methods. Through large scale online exper- iments, we show the effectiveness of the proposed approach, and as a result, the benefits it brings to the whole market- place including both LinkedIn and advertisers. The achieved benefits can be characterized as: 1) simplified targeting pro- cess and increased reach for advertisers, and 2) better uti- lization of LinkedIn’s ads inventory and higher and more efficient market participation.