That sucks, but unfortunately your best bet is probably to keep quiet and not make a fuss about it.

Obviously DocuSign has looked at the HelloSign experience and decided that it's good. That's great, because it means you've had 18 months' headstart on your competitors with a more effective onboarding experience. You can't ask for better validation!

You can't build a SaaS service and expect your competitors to sit back and not copy your best ideas. There's no actual IP infringement here, so there's nothing you can do, and that's the nature of competition!

I can't blame you for a bit of snark, but honestly – just drop it. Complaining about competitors copying, and trying to drum up a social media campaign, comes off as extremely childish at best. Just focus on building a better product, and leave it at that.