It seems that there's a big spike in fake profiles on Linkedin. These profiles are used by scammers to get information out of you that they then exploit for financial gain. This technique has been called "Social Engineering" for a long time; the fact that it's been around for a long time tells you all you need to know about its effectiveness.
Linkedin has a good (despite being rather old) blog post about how to spot a fake profile.
My attitude has always been to ignore requests from Linkedin users who I don't recognize, and who don't craft a personalized invitation pointing out how I've worked with them (or someone I know) in the past. "I'd like to add you to my professional network" messages from folks I don't recognize go straight to the bit bucket.