When it comes to dating, women from ages 25-30 have many options. Due to our social setting, men usually will relentlessly approach those girls at bars, workplaces. Most of them will not settle with a single partner until they think they are ready. Time passes and they eventually are ready but there is a problem. The horde of men that once approached young women eventually settled with one and are in a long-term relationship now. Yes, this also happens to pretty boys, but given that men are socially more motivated to 'get girls', the prettiest girls seems to be the more affected by this behavior. Specially because the average girl will sense they have a slight disadvantage and will try to settle faster than the most popular ones. (obviously a complex matter, you can read more about it here.)
The lonely pretty girl syndrome is exactly what happens to many startups/websites. You have a good idea, maybe a decent execution and decides to deploy an 'invite only' version of your website. Those are the consequences: people who you managed to get a little attention will move on, desire is a very limited thing, specially for services. When you finally invite those people they won't sign up. There was a very small number of users that really wanted to use your service, but you dismissed them along with the horde of curious hipsters, who just wanted to see the website, tweet they have an account and move on with their lives. I can only come up with three reasons for startups doing invite-only websites: 1) They are testing if there is a market. There's no product, that's just a screenshot. 2) Their implementation is crap; 3) They want to give a sense of exclusivity, add some kind of 'desirability', which is probably a very bad idea.
Today I was presented to a new website which seemed pretty nice. It is somewhat similar to Hacker News, but with an improved interface. The articles posted there were also pretty similar those you would find on HN. So I decided to submit a post I wrote weeks ago but I couldn't - that's a invite-only website. The same thing happened months ago: I tried to create a blog in website called 'subtle' but it is a 'curated' website and I was probably rejected, weeks later I ended up getting an email asking with I was still interested, even though I said 'yes', I never really created any blog there, I had just signed up to squarespace, which is great and makes me very tasty meals.
Hope you had fun reading my non-sense.