One commonly mentioned danger of a large corporate player like Twitter coming into the decentralized social world is the Embrace-Extend-Extinguish tactic, wherein one entity adopts a common protocol, and once it acquires a large marketshare then makes itself incompatible through custom changes, locking its users in and leaving the common protocol for dead.
Something like this happened to another federated/decentralized technology, XMPP. XMPP was used by Facebook Messenger, WhatsApp, and Google Talk to power instant messaging, then one by one those platforms disabled federation features and now barely anybody still uses XMPP. People fear something similar is going to happen to e-mail through GMail.