AOL used SMTP and NFS and TCP/IP and many many other open protocol interop things. It wasn't great at sharing back, I am afraid, but it wouldn't have been able to be what it was without the internet protocols and many other open things (network socket programming, DLPI, heck sendmail, SSL, HTTP compression). AOL is a prime example of how a solid infrastructure enables new businesses (but those businesses might not stick with the partner that brought them).