Twitter is probably the only tool among these three that will survive because it is contantly adapting to the new things that competition releases but the disadvantage is that it is harder to interact with people and it is even harder to find information.
Myspace on the other hand is dead with just 1% of the traffic it had back in 2007 and this number is going down. What happened with Myspace? It failed to become simple, fast and effective. Myspace had graphics all over the page, it was too slow to open, it failed to listen to what people wanted and in return people just left to Facebook.
Delicious used to be a great site that had less information than Google but its quality was enhanced because it was people driven and only the good pages would get enough likes or shares, call it whatever you want, to rank at the top and there was no way to beat the system. The problem is that people are now getting information from Facebook and less information from Delicious.
Which of these three tools will be in business five years from now? I believe that only Twitter will be around for that long. Maybe Google Plus will be so successful that no other social network will have a market to conquer.