Facebook direct targeting and lookalike audiences are a good option to start. This wouldn't be violating terms. Google is launching a look alike audience tool soon.

Depending on your product, you may want to build an automated direct marketing approach with a tool such as www.close.io. This approach would be your least expensive and most cost effective. I've done this at scale myself. It's incredibly effective.

I'd be happy to chat to help you get going.