Now you might ask me - who uses Bing? Well I do, and Microsoft claims 5.5 billion monthly searches on its search network and that's pretty big. Furthermore, Microsoft Ads is less competitive simply because it's not as used as Google. Now go on and have your ads shown on Bing search results.
LinkedIn is a completely different social media ecosystem, as it's a professional social network. But advertising on LinkedIn is pretty much like Facebook, but with an audience that's comprised of mostly working professionals.
Another underrated advertising platform, Twitter ads work like Facebook. What's more, you can also pay to promote a trend. According to Twitter, a Promoted Trend is a "24-hour high-impact takeover of the Trends list on Twitter", so in essence you get to be on top of the top trends for a day.
Do share any other ad platforms you've had prior experience!