Do you believe that new social network can be created rival to others? what kind of new idea may be header point of new network? or you think that facebook, twitter and google + has already did what can be make? sorry for my bad english.
Eventually Facebook and Twitter will decline in popularity and something else will take their place. Remember what happened to Myspace? What will replace them? I would love to know.
I sort out some points why social network like MySpace decline. Major reason for MySpace's failure was: lost control on their advertisements and creating a very unpleasant user experience ridden with ads. MySpace allowed raw HTML coding which gave the ability to embed malicious scripts within the page. MySpace decline is they raley evolved the services; it became stagnant and boring. I guess the lesson here is simple, “once you’ve got it up, keep it up,” at least in the world of technology.
I guess it would be very hard to for another social network to enter the market now. From the technical point of view, Google+ is just fantastically built. Facebook is synonymous with "social network". Anyway, if you decide to build one, make it lucrative for social marketing as well. If businesses join, they'll be promoting the network while promoting themselves. Maybe a network for teens? I guess you'd need to find a unique focus for it.