[netcan]

"Replace" is a loaded term in the sense that it implies these are categories dominated by one player. I think the reality of these kinds of services is that they overlap each other to various degrees rather than replace. People can use facebook, twitter, linkedin, HN, etc. etc. simultaneously.

So if you mean will other services in that general fuzzy category become popular, the answer is 100% yes. It's happening all the time. Whatsapp, snapshot, whatnot. These pop up all the time and many get traction.

If you mean will Twitter & Facebook loose popularity, that's harder to answer. But, I think it's reasonably likely. They're basically mediums for online culture and online cult evolves fast. It also depends on your horizon. If the question is "will a large portion of humans check Facebook daily in 100 years?" that seems pretty unlikely.