In the short term, I think it's unlikely that online personalization will be accurate enough to create the type of bubble Tim Berners-Lee is suggesting.
The type of bubble TBL is suggesting is actually just the reddit hivemind. It already exists. People like it.
It seems to me that he's talking about computers personalizing based on their interactions with you passively.
However, I completely agree that reddit and almost all website that brings together like minded people will have this "bubble" effect. That, however, is nothing new. It's the same as similar groups of people going to the same church, or joining book clubs, or gathering in groups for any reason. The scary thing here is sites that everyone uses will begin to passively function in the same way, making it even less likely than ever for someone who isn't seeking out a diverse range of opinions to find them.
Also worth noting is that there is a moral assumption here that being exposed to a diverse range of opinions is a good thing. If you don't believe that, then this is a moot point.
The type of bubble TBL is suggesting is actually just the reddit hivemind. It already exists. People like it.