Has the sun begun to set on Twitter? Has its finest moments faded into the past?

It's probably too soon to sing the company's death knell, but there are disturbing trends afoot that suggest times are getting increasingly tougher for the nano-blogging site trying so hard to maintain some sense of relevance.

The latest metrics from comScore paint a rough picture for Twitter: Americans using the site decline 7.9 percent in October, compared to September, signaling the second monthly decline for the site in 2009. Yes, the site still boasts 19.2 million users, but the company's growth of less than 1 percent in September, and an actual decline tracked in August is a real problem.

Why? Because even during the dot com bubble Web sites like Pets.com and Webvan and so many others that litter the Silicon Valley graveyard found ways to create at least the appearance of growth. Instead, Twitter continues to drift through the marketplace like a dandelion seed, going where the winds of trend and word-of-mouth and hype take it.

We've certainly seen some interesting deals involving Twitter. Sharing data with Google

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[GOOG  Loading...      ()   ] is intriguing, and TV commercial I saw on our air earlier this morning from mattress maker Tempur-Pedic directing potential customers to Twitter might hold promise, but is that it? Really?

Twitter runs the risk of devolving into another marketing tool by companies looking for what might seem like a cool way to speak to customers. But if that's all it is, I'm not sure how many customers will be left to hear the message. And where's the beef? Where's this company's revenue-generation strategy, or Heaven forbid, profits?

I don't argue that Twitter has a big base of users. I just wonder how many of those 19 million are passionate enough about the service to stick with it beyond its initial attraction as a curiosity. And how many of those 19 million are going to spend some hard-earned scratch on the service, or on those who advertise with it or market through it.

Also, new rounds of venture capital funding do not equal "revenue" growth.

I don't know, but it seems as though Twitter is running the serious risk of so many dot com deaths that came before it: Trying to identify a market for its service after it's already in business, rather than identifying an unmet market need and going after it.

The decline in Twitter usage is more than an irrelevant data point as a kind of snap shot of today's activity without deeper ramifications. The comScore data about Twitter should be watched very carefully, and taken very seriously. I'm not sure how long these guys can last as an independent entity and with numbers like these, its valuation will now be under serious pressure.

Shhhhhh, listen: Hear that hissing noise?

Questions?  Comments?