Often these alerts are formatted to appear official, and contain details about security problems and require you call some provided number to free your system. Sometimes they may outright demand a ransom to fix your system. This is a common trap used by cybercriminals to convince people to hand over money, and is being used by interested parties to block access to the MacKeeper settlement options.
This approach is often taken when new malware removal techniques and tools are published online. Unfortunately it means that those who are in need of the information or tools located at the URLs will not be able to get them.
Things are starting to get a bit nasty now for mac users . . . Be Aware !