You bring up a great point about Reddcoin. Even if as @knircky suggests that most other cryptos don't yet have a working platform, it's just a matter of time before competitors to Steem start emerging and if Reddcoin implements what they have set out to, it will enormously reduce the potential for Steem to grow.

Of course even if Reddcoin works, FB/YT/TW will be be centralized companies, contrary to Steem whose Unique Selling Point is that it is decentralized.