Google+, which is the most flexible of all the social networks, can be used as an interface for Twitter and Facebook. All the social networks enable you to be notified by email or SMS. And tools like Seesmic Desktop give you a dashboard to all your social networks.
So options exist that save you from having to constantly log in to multiple services all the time.
Another problem is that MySpace isn't considered an option by many of the people it's best designed for. The service has failed to evolve and adapt in the past few years (OK, New Corps actively damaged MySpace through incompetence), so members are still leaving.
MySpace is currently owned by an advertising company called Specific Media. Singer and actor Justin Timberlake is also an "owner," but this looks more like a kind of sponsorship-driven publicity stunt than an investment.
As a result of the failure of MySpace to innovate in the past few years, the Party Animals have been set adrift, showing up on all three of the other major networks.
From an industry perspective, there's a big opportunity still for someone to create what MySpace should be -- a gathering place for the Party Animals who want to communicate with each other through impressions and style, rather than reasoned discourse, and one that revolves around music and popular culture.
(Personally, I'd love to see MySpace founder Tom Anderson buy it back and resurrect it.)
There's also a potential trap for the other networks, who will be tempted to be all things to all people, and ruin what would otherwise be a good thing.
The best social network is the one that specializes in one of the four social personality types.
This is best for all concerned, by the way. If you're, say, a Google+ Idea Conversationalists, you want the Party Animals to have somewhere else to go. And if you're a Party Animal, the last thing you want is some Debbie Downer dragging the party down with boring chatter about some TED video.
In the meantime, let's give the social network-killer talk a rest. Not only is the dominance of a single social network unlikely, it's also undesirable.