General practice guide for developing with HappyCha.in API
Sign in with Facebook
The blockchain IS a database, so you don't need to host your own. Instead of saving user credentials, use Sign in with Facebook to get unique ids for your users.
While most people don't know what the blockchain is yet, some power users do want the ability to interact with your smart-contract without your help. Options already exist for letting people use your smart-contract without using your app, the easiest of which by far is a Chrome plugin called MetaMask. To allow your users to have a range of control over their wallets, you should build your Ðapp to support MetaMask users. (Bonus: MetaMask users pay for their own gas!)