The more often we encounter a brand, item, or person, the more likely we are to develop a liking. Whether it is a company, a colleague, or a car, if we spend more time with something or someone, our minds become more familiar. This familiarity leads to trust, which then leads to liking.
The more often someone is exposed to your brand, whether it is a billboard or social media ad, the more familiar they will become with it. Over time, with enough exposure, they will have a higher and higher likelihood of engaging with you. Be careful not to annoy them, however. Too high of an ad frequency can lead to outright dismissal of your brand.