Final answer:
The central feature of early online communities was the message board system, which was key in fostering local digital social interactions.
Step-by-step explanation:
The primary feature of early online communities was message boards, also known as bulletin board systems (BBSs). These BBSs were pre-internet networks that used personal computers connected via modems and phone lines. Similar to contemporary websites, they allowed users to share files, post messages, engage in discussions, and play text-based games. While features like chat, network discovery, and widgets became significant later, message boards were the cornerstone of early digital social interaction, particularly because they catered to local communities due to the expense of long-distance phone calls at the time.