Donation wallet address
I read that wallet/account is not the same as bitcoin address. 


A bitcoin-wallet is only a way to store multiple private keys (corresponding to multiple bitcoin-address). 


  The address can be used only for single transaction. How do you receive donations on your address?


Addresses can be used for an infinite number of transactions. If you put a donation button somewhere referring to an bitcoin-address of yours, all those donations will get paid to this one. 


  Are wallet addresses unique for the network so that I can generate my wallet in one service, 
  eg. blockchain.info, and used it in different service?


With a wallet (like the bitcoin-core client) you can generate new bitcoin-addresses as well as using other services like blockchain.info. Those addresses are unique and by exporting/importing the private key you can use those addresses in other services as well.