Last month Facebook apparently offered Twitter $500m worth of its stock to buy it. Mark Zuckenberg is a known fan of Twitter, having described its ‘model’ as ‘elegant’ at this year’s Web 2.0 Summit.

Kara Swisher cemented the acquisition rumours in an article subtitled “The Deal That Fail-Whaled”, referencing Twitter’s infamous ‘out of action’ whale image (now on the endangered list following some technical improvements).

The reason why the deal failed? Team Twitter didn’t buy into that $15bn valuation, which Facebook acquired after an investment from Microsoft in October 2007.

Twitter has rightly questioned Facebook’s value, given that the valuation is effectively more than a year old. A lot has happened since then, particularly at macro level (note to Facebook: we’re about to enter a recession).

In the past 12 months Facebook may have seen a six-fold growth in usage, but – and here’s the kicker – not in profits. In fact Facebook is said to be losing money, and losing big. And despite raising more than half a billion dollars, it may need to raise yet more. That’s going to be a telling time for the company and it’s self-assurance, because I can’t see how that $15bn valuation will hold up if a new investment round materialises.

I have previously argued that if Facebook thinks it is worth $15bn then it is probably worth about $5bn, based purely on market trends. It’s the only thing I have so far agreed with Henry Blodget on, ever.

An article on Venturebeat earlier this month wagged a finger at Digg for ‘not making any progress’, despite the fact that its valuation has pretty much held up in a tricky market (it is worth about $164m, based on a recent investment). But can Facebook really maintain a nine-digit valuation? I doubt it. And here’s why: the market has dumped on tech stocks in the past year, despite – in many cases – surging profits and revenues.

Take Google, for example. The poster child of the internet reached a high of $724 in the last 12 months. At the close of play on Friday it had slumped to a mere $262 per share. Almost two thirds less than $724. Google hasn’t done too much wrong to merit this fall, but market sentiment is apparently worth at least 50% of a company’s valuation. There may be a way to go yet (Dow to 6,000, anyone?).

If Facebook really believed in its valuation, had access to funds, and seriously wanted Twitter, then it could offer $250m in cash and I’d be dumbfounded if the sale didn’t go through.

Strategically, and cash permitting, both companies might do better to hunker down, preserve cash, and wait it out.