I highly doubt they will ban cryptocurrencies from their ad-platform when they can barely stay afloat. I speculate Facebook did so only because they're under pressure plus Zuckerberg said he's looking into the blockchain. I wouldn't be surprised if they'll monetize Messenger or WhatsApp with (the speculation it self of) a token in 2019/2020, akin to Telegram. Messaging-based apps are notoriously hard to monetize and Zuckerberg has said on earning's calls that he's unsatisfied with Messenger's progress in terms of monetization.