Startups can use a variety of strategies to build an audience quickly, including:

1. Leveraging social media: Startups can use social media platforms to reach a large audience at a relatively low cost. They can use it to create and share engaging content, interact with customers and build a loyal following.
2. Influencer marketing: Startups can partner with influencers in their industry to reach a large, engaged audience and gain credibility.
3. Paid advertising: Startups can use paid advertising on social media platforms and search engines to reach a targeted audience quickly and efficiently.
4. Content marketing: Creating valuable and informative content relevant to the target audience can help establish the startup as an authority in their industry, increase visibility and attract new customers.
5. Public relations: Startups can use public relations to gain exposure and credibility through media coverage and press releases.
6. Email marketing: Startups can use email marketing campaigns to reach a targeted audience and build a relationship with them.
7. Referral marketing: Encourage existing customers to refer their friends and family to your startup, this can help to quickly grow your customer base and increase trust in your brand.
8. Search engine optimization: Optimizing the startup's website and online profiles for search can help them show up when customers are searching for products or services related to their startup.

By using a combination of these strategies, startups can build an audience quickly and efficiently.