Facebook has walked away from talks to buy Twitter because the $500 million price tag was too high, according to a report by AllThingsD.

For Facebook to be a picky value shopper is quite a turnabout from just two years ago, when Yahoo walked away from a deal to buy Facebook largely over the price.

I don’t think that Twitter is as important to Facebook as Facebook could have been to Yahoo. But the whole thing shows how hard it is come up with a value for this sort of Internet company.

Frankly, so much of the commentary about the “value” of these companies assumes there is some sort of calculation that will say whether Twitter is “worth” $500 million or $150 million, or whether Facebook is worth $1 billion or $15 billion. These are not utilities with predictable schemes of cash flow that can be discounted using an H.P. calculator. They are better seen as lottery tickets — companies that just might capture a lot of user attention in some specific area and then might find a way to profit from that attention.

Two years ago, Yahoo had to evaluate a young Facebook. It was rapidly growing, yet it had unclear prospects for revenue and it was unclear whether the site was more than a fad. Ultimately, Yahoo decided the company wasn’t worth more than $1 billion.

I’d argue on balance that Yahoo made a mistake. Even though Facebook hasn’t gotten the profit part of the equation down, it turned into an important platform — the sort of site that people use to organize a lot of their communication and information. And most significantly, it has become a direct rival to Yahoo.

Yahoo’s decision to pass on Facebook wasn’t quite as disastrous as its decision not to buy Google early in that company’s history. On the flip side, Yahoo, like lots of other Internet companies, has certainly done its share of dumb deals, like buying Geocities and Broadcast.com. Picking out a Google from a Geocities is more art than science.

Now Facebook is presumably looking at Twitter much the same way — as a company with no revenue that nonetheless could be important. Twitter is surfing a growing wave of user behavior and engagement that is related to, but not the same as, how people use social networks, blogs, cellphones and other tools of self-expression. The wave may crest and Twitter may stumble. Then again, Facebook could find that Twitter by itself, or as part of a bigger company, could help shape a rival community of social sharing.

If the public reports are to be believed, Facebook decided that the answer to these questions justified giving Twitter 3.33 percent of its stock. That’s worth $500 million if you accept the $15 billion valuation for Facebook that was set when Microsoft invested in the company a year ago.

But no matter what you think Facebook was worth last year, its value must have fallen since then simply because the share price of other Internet companies have fallen. (Google, which does have revenue, has seen its stock fall 61 percent over the last year.)

I’ve got to imagine that Twitter’s management and investors are torn. Given the economic storm clouds, it has to be tempting to take some money off the table. Then again, they have a hot hand now, with Twitter taking off among a wider range of users, so there is the temptation to double down and see how much they can win.

My bet: the two sides won’t come to an agreement on price anytime soon. And in any case, it will take years to figure out whether Twitter is more Geocities or Google.