Some of us remember the dot-com bust at the end of last century. I was finishing my studies in New Media. Graduates were being offered salaries that only a senior designer could command today. It was all a little ridiculous.

Is there another dot-com bubble getting ready to burst?

Sub-prime mortgage crisis...finance company collapses...mortgage rates going up...while these events may not be directly linked with the dot-com bubble it should serve as a warning that investors will be more wary with their money and will require a return on investment.

Last week Business Week reported that Facebook staff were beginning to sell their stock, at prices below what the company was valued at after Microsoft's purchase of shares in October 2007. Then the company was valued at US$15 billion. Today the price of shares being sold suggest a value of US$3.75 to $5 billion. That's a big drop.

While there isn't an investor flight away from Facebook it wouldn't take much to start one. The company has had a hard time trying to monetise it's investment. Revenue for 2007 totalled US$150 million. It's predicted revenue will be US$300 - $350 million in 2008.

Facebook was touted as the next big thing in the world of social marketing. It might be for the users, but it proves to be anything but for the investors.

Now Fortune has interviewed the inventor of Twitter - the next, next big thing. Adam Lashinsky starts his column:

I am sitting in a meeting room at the San Francisco offices of Twitter, chatting with the fast-growing startup's 31-year-old CEO, Jack Dorsey, when a wave of déjà vu washes over me. The youthful vibe, the playful decor, the funky South of Market loft space - I've been here before. In 2005, Mark Zuckerberg earnestly explained to me the importance of Facebook as we sat in his similarly appointed office in Palo Alto. Chad Hurley and Steve Chen walked me through YouTube's growth story the following year in their cramped space above a San Mateo, Calif., pizza parlor.

Facebook and YouTube have yet to gush profits - a fact that is the talk of Silicon Valley. Yet here I am again, in July 2008, listening to yet another boyish entrepreneur discuss a quirky, compelling - and nearly revenue-less - startup.

 And later in the piece:

Twitter's CEO won't go into any detail on how it will evolve from hip technology to moneymaker. "A lot of people have ideas about how to monetize this," says Dorsey. "And so do we."

Twitter can claim millions of subscribers, but how many of those subscibers are active users? I'm a subscriber but I've never used the service. If I check the people who have gone to the effort to 'follow' me on Twitter none of them are active users either. You can't make money out of inactive subscribers.

When does the tipping point at which investors demand a return on their investment occur? Is it happening now for Facebook? If Facebook collapses what will be the implication for other social networking sites?  And how do you monetise a social network which began as a response to over-advertised environments?