Here are a couple of links to 501(c)3 compliance guidelines – specifically for social media. They’re a bit dense — but are worth skimming through.

• Bolder Advocacy: LegalTips On Using Social Media For Advocacy 
• Bolder Advocacy: Influencing Public Policy in the Digital Age:The Law of Online Lobbying and Election-related Activities

The biggest takeaway is that 501(c)3s are “nonbias” – we can support causes, and lobby elected officials to support them (or critique their stance). However, we cannot directly endorse candidates (address the issue, not the person). This is much easier during non-election years/periods!