I have seen them on MySpace, Friendster, Facebook and here on LJ. I inadvertently clicked on one of the links last year and was infected, fortunately my anti-virus software caught the problem immediately. However, others weren't as lucky and have had their entries deleted, among other things.
One of the many ways hackers get people to click their links is by saying that a particular community is closing. Let me assure you guys that we have NO intention of closing this community. linguaphiles has been around for 8 years and we intend to be around for as long as LJ is.
In any case, I suggest reading the post (link: http://community.livejournal.com/ljsecret/369273.html ) which provides helpful info.
Basically, be vigilant about the links and embedded videos and pictures that are posted - if you're not sure, don't click on them. If you see such content, alert LJ abuse as well as e-mailing our YahooGroup which reaches all the maintainers.