Twitter is now beginning to see a substantial rise in active users. A recent report found that the percentage of Twitter users who have tweeted ten or more times, have more than ten followers, and follow more than ten people rose from 21% to 29% in the first half of 2010.

Spammers, scammers, and thieves are paying attention.

In the physical world, when communities become larger and more densely populated, crime rises. This also applies to online communities, like Twitter and Facebook.

Twitter’s “direct messages” and “mention” functions are laden with spam, often prompting users to click various links. Why anyone would want me to “Take a Good Look at Hypnotherapy” is beyond me, but someone must be buying because the spam keeps coming.

Common Twitter scams include:

Hijacked Accounts: Numerous Twitter (and Facebook) accounts, including those of President Obama, Britney Spears, Fox News and others have been taken over and used to ridicule, harass, or commit fraud.

Social Media Identity Theft: Hundreds of imposter accounts are set up every day. Sarah Palin, St. Louis Cardinals Coach Tony LaRussa, Kanye West, The Huffington Post, and many others have been impersonated by fake Twitter accounts opened in their names.

Worms: Twitter is sometimes plagued by worms, which spread messages encouraging users to click malicious links. When one user clicks, his account is infected and used to further spread the message. Soon his followers and then their followers are all infected.

DOS Attack: A denial-of-service attack left Twitter dark for more than three hours. The attack seems to have been coordinated by Russian hackers targeting a blogger in the Eastern European country of Georgia.

Botnet Controller: One Twitter account produced links pointed to commands to download code that would make users’ computers part of a botnet.

Phishing: Hacked Twitter accounts are used to send phishing messages, which instruct users to click links that point to spoofed sites, where users will be prompted to enter login credentials, putting themselves at risk of identity theft.

Twitter Porn: Please, “Misty Buttons,” stop sending me invites to chat or to check out your pictures.

Twitter Spam: The use of shortened URLs has made Twitter’s 140 character limit the perfect launch pad for spam, shilling diet pills, Viagra and whatever else you don’t need.

To prevent social media identity theft, take ownership of your name or personal brand on Twitter. Protecting yourself from other scams requires some savvy and an unwillingness to click mysterious links. In other cases, you’ll need to keep your web browser and operating system updated in order to remain safe. Make sure to keep your antivirus software updated with the latest definitions, as well.

Robert Siciliano, personal security expert contributor to Just Ask Gemalto, discusses hacking wireless networks on Fox Boston. (Disclosures)