We need some kind of service where we could store our savings (making them generate profit). This is usually called BANK. So we store our bitcoins in an account, and the bank lends these bitcoins to someone who want to invest. This person pays interest to the bank, and the bank pay us that interest.  Of course the bank should only lend the money for the period of time we have agreeded to save it.

This is not a fractionary reserve system because I cannot spend my bitcoins while they are lended to someone else.