All language is invented, every time somebody coins a new word they're inventing language. What doesn't work is to raise the barrier so high that people will have to learn a lot of new words at the same time.
Learning language is an incremental thing, and the pressure to learn it is dependent on the installed base.
An artificial language will be hard to spread because there is the chicken-and-the-egg problem, if enough people spoke it then there might be a reason to cross that huge barrier to entry.
Words aren't language; an example of a "natural" languages invented today can be found in http://en.wikipedia.org/wiki/Nicaraguan_Sign_Language. Many creoles follow the same pattern of spontaneous development. This is a distinct difference from the inventedness of Esperanto or Lojban.
Artificial languages don't spread because they don't solve anyone's problem; they solve everyone's problem.
Learning language is an incremental thing, and the pressure to learn it is dependent on the installed base.
An artificial language will be hard to spread because there is the chicken-and-the-egg problem, if enough people spoke it then there might be a reason to cross that huge barrier to entry.