The software works or it doesn't. This limited membership pool is ridiculous. People will be talking about Google+ for a few days (maybe a couple weeks). If Google does not open up membership to the general public in that time, it will die. I remember when Gmail invites were the next big thing. I didn't even notice when Gmail opened up to the general public (took many months), all I know is that Hotmail and Yahoo! Mail still have more accounts than Gmail. If Google wants to compete with Facebook, it needs to be there right away. Unlike email, social sites like this are living breathing organisms. You need those members to keep the website alive. You can TRY to be elitist, "we're only letting so many people in", to try to build up buzz, but when people can (and already do) use Facebook instead, why are they going to hang around trying to get into Google's exclusive club?
Face it, people use MySpace OR Facebook OR any of the other alternatives (especially regional ones), but unless you are a business or celebrity, how many people use more than one? How many millions of dead myspace pages are there? Those people didn't disappear, they just moved on. Google needs people to move on from Facebook for Google+ to be a success. Those people won't even try if Google won't let them.
I get an email from an acquaintance now and then inviting me to join Facebook, and you know what, if I cared, I could click that link and actually join. You try to join Google+ today, and you'll be told to come back later. First rule of sales is never let the customer walk out the door without buying something, and Google won't even unlock the doors to let customers in.