Bitcoin is actually managed by a decentralised network of participants called "nodes". These nodes run the Bitcoin protocol on their machines and maintain a record of the entire Bitcoin blockchain.
No single entity, company or platform manages Bitcoin. Platforms like Bitstack provide services that allow users to buy, sell, store and manage Bitcoins, but they do not control the Bitcoin network itself.
Bitcoin is the only cryptocurrency without a company or foundation. This is one of Bitcoin's strengths: its decentralised nature prevents any single entity from taking total control of the network. As a result, no single entity can manipulate the ledger of transactions, which is secured by the combined computing power of all the nodes on the network.
Who takes care of improvements and updates to the Bitcoin protocol?
Bitcoin's code is entirely open source. It is reviewed and improved in an open and public manner by volunteer developers from all over the world. Improvements proposed by developers must be validated by the network nodes and implemented collectively to apply to the network.