How do I wipe everything out of an old iphone?

This will answer the question you asked→What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support, but it may not solve the problem that you are reporting. You should be able to use social media apps from multiple devices simultaneously. An exception might be apps such as WhatsApp, which are tied to your phone number. You would need to consult the support site for those apps to change your device.