Both sites have lately become attractive targets for online crooks who try to trick users into installing malware on their PCs. Malicious tweets or Facebook messages might , but instead install fake security software. Kaspersky today posted about the most social engineering attacks used by the against Facebook and Twitter users.

But today's attacks against the sites are different. Denial-of-service attacks, or DoS, overwhelm a site or service with so much garbage data that it can't respond to normal requests. Your browser might ask a site to send the text, code and images used to display its Web page, but during a DoS attack the site is snowed under and can't respond. So you don't see anything.

These types of attacks are usually distributed attacks, or DDoS, meaning the flood of data comes from many different sources. Usually that means a botnet - a botnet controller can order every infected PC in the botnet to send garbage data to a particular site. When a given botnet might comprise tens or hundreds of thousands of bot-infected PCs, that ends up being more than enough garbage data to overwhelm many sites.

But while crooks can and do make a good deal of money with malware and fake security software, and use any number of tricks to get it on PCs, denial-of-service attacks don't install malware. They don't steal data that might be sold, and while DoS attackers have in the past used the assaults to demand a ransom from the victim sites, there's no word of such a request from either Facebook or Twitter. And large sites aren't usually the target for such (already uncommon) extortion attempts. So why are they happening?

Malware wants to quietly steal and make money, but DoS attacks are generally meant to hurt their target in a very public fashion. And one commentator, Randy Abrams of ESET, which makes antivirus software, guesses that Twitter in particular might have by fighting back against malware attacks. He notes that Twitter has recently begun filtering URLs to block those used in malicious tweets, which might have hurt the crooks' bottom line.