People managed just fine in the past. Word of mouth. Fliers. Set up a lightweight static website. You don't need 70k people to be able to press a 「いいね」 button or leave a comment. People can use text messages, IM, email, and their own facebook groups, twitter groups, or whatever to spread the word. You can keep physical ledgers of people you organise with.

There's no reason that a group of 70k people couldn't be thousands of groups of handfuls of individuals all working for a similar cause.