Most malware implements communication with their C2 server over HTTP(S). Why? Just because it works! But they are multiple ways to implement C2 communications: DNS, P2P, Layer 7 (Twitter), … Another one that has become less popular with time is SMTP (email communications). I spotted a malicious Python script that exchanges information with its C2 server through emails.
Article Link: InfoSec Handlers Diary Blog - SANS Internet Storm Center