I don't know if blockchain.info and coinbase.com are the most used web wallets but I know that me and to many others I know have both wallets. I use mostly coinbase because with it doesn't pay fee when I send bitcoins but I have another in blockchain too. Iknow even that coinbase is used by million of peoples (I don't remember the exact number but i know that are millions). Counting that bitcoin is not known to much these figures are the most significant figures that testify that both those wallets are the most used wallets for the bitcoin.