The dark days of Twitter spam — or "Twam," to some — are about to descend upon us.

The rapidly growing social network, which lets you communicate in 140-character "tweets" to others who "follow" your every move, has been infiltrated by an influx of unsolicited advertising.

But to some, there's a big difference between legitimate firms using Twitter as a helpful marketing tool, and the fly-by-night onslaught of messages that promise to enlarge breasts or other body parts.

Twitter's own half-hearted attempt to control the spam has had little effect. Third-party Twitter clients are racing to come up with better solutions, but as businesses rush to use Twitter, some experts warn that the worst is yet to come.

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Technically, says Christopher Peri, CEO of the Web-based third-party Twitter interface Twittfilter, what we're now seeing is really Twitter spam 2.0.

"The first iteration of Twitter spam was a tweet that showed up with a picture of a pretty girl on it," he said. "You looked at it and realized you just got spammed."

Peri also points out that that was a hugely inefficient way to send spam. It required a lot of manual work for the sender and was fairly easy to block, which is exactly what happened when early Twitter adopters began reporting the problem back to Twitter's home base in San Francisco.

The second phase of Twitter spam, which we're seeing today, is a lot harder to identify, and thus a lot harder to protect against.

By using computers to hunt for keywords and replying to "relevant" usernames, spammers are dumping their Tweets into your "timelines" — recent Twitter posts by yourself and everyone you're following.

The number of such unwanted tweets is rising. What was once maybe one or two messages a day has now risen to around 10 percent of everything that's showing up.

The problem with Twitter spam, as opposed to, say, MySpace spam, is how personal it can be.

Peri tells a story about a couple of friends at a baseball game who were sitting in separate sections. They used Twitter to figure out where to go for pizza afterward.

"One guy wanted to go to one place. The other was lobbying for a different restaurant when a Tweet showed up suggesting they try a third option," said Peri. "The sender of this Tweet was the owner of this third option. He promised these guys a 10 percent discount if they came by his restaurant.

"The guys went there instead and had a great time. So is this spam — or effective marketing?"

Peri's opinion is that spam is any message he doesn't want to read.

Yet other established businessmen, such as author, Entrepreneur Magazine columnist and all-around tech guru Guy Kawasaki, have been very vocal about using Twitter as a marketing tool. To them, Twitter spam is simply a good business practice.

"I may get more value out of Twitter than anyone else on the planet because I use Twitter as a tool — specifically as a marketing tool," Kawasaki blogged back in December.

"If the concept of using Twitter in a commercial manner interests you, keep reading," he wrote. "If it doesn't, then you can continue to send and receive tweets about how cats are rolling over and the line at Starbucks."

Kawasaki's message, which he promotes often, is starting to sink in — which could have very wide-ranging effects.

Currently, if you write a Tweet asking for airline suggestions, you might be "followed" by JetBlue or Southwest, both of whom make good use of the network.

But as more businesses adopt this practice, forcing their competitors to jump in as well, you may start seeing every airline around clogging up your inbox with 140-character ads for cheap tickets.

Peri, whose Twittfilter is one of a number of third-party Twitter interfaces, counters spammers and marketers in Twittfilter by applying a scoring system completely personalized to a user's circle of friends.

That makes it much harder for spammers to break into your network.

"Twitter is only as vulnerable as the network you create," says Peri. "If you're using the site as it was originally intended — as a way to stay in touch with close friends and family — then this is the perfect filter."

But if you're a power user, following hundreds of Twitterers throughout the day, then Twittfilter isn't going to work for your needs.

To this end, Loic Le Meur, whose company owns the third-party Twitter client Twhirl and "Twitter of video" Seesmic, is now adding a "report as spam" button to Twhirl.

That's a major improvement on how Twitter currently handles spam, which entails sending a direct message to "@spam," an extra step most people wouldn't bother to take.

Twhirl's "report as spam" button automates and simplifies that process. If enough people click it to report a sender as a spammer, then the folks at Twitter headquarters can quickly block him.

Twhirl's button won't free you from spam completely, which Twittfilter's more general filter almost does.

But it's much more acceptable to corporate Twitter users, such as Dell and Comcast, who have been using the service to communicate with customers and who wouldn't want to be blocked by a general filter.

(Le Meur says if you want to see how much attention they're really paying, try writing "Dell Sucks" in a tweet and seeing how long it takes them to respond).

Le Meur is sympathetic to such business users, since his own Seesmic Desktop gets about one tweet a minute from potential customers.

Peri is too — to an extent. And he emphasizes that in such a new medium, the users will get to decide.

"People need to make up their mind about questions of free speech, personal space and unwanted marketing," he says.

He points out that until the American public reaches a decision on what qualifies as spam and what doesn't — and starts to legislate tougher laws for offenders — it doesn't matter what new networking system shows up.

"Until the stick surpasses the carrot," says Peri, "we're going to be fighting this fight."