The best way is to use key value stores rather than SQL as SQL centralises data for sure. It’s a shift for many db devs for sure, but it is worth it. Then you can have db’s that are enormous, but you may shift from immediate to eventual consistency as amazon, google, facebook etc. do at the moment.
SQL can be useful for smaller sites and uses, just as in the server based world really. I posted a video here recently by a chap called Fowler that explains it all really well. So it’s best we provide all those capabilities, but an SQL only solution is likely to be the wrong solution in my mind anyway.