Eh it’s a tool like anything else.
Can be used for good or evil.
It is extremely helpful for anything requiring a mass response, like planning a large event, responding to someone who had a disaster or death in the family, a prayer group, etc. Getting 50 RSVPs or condolences in your regular e-mail box is a nightmare trying to sort them out from the bills, the spam, etc. I avoid friending anyone I do not know personally at this point, although many of my friends I did meet on the Internet pre-Facebook and the friendships have endured through three or four changes of the “trendy” tech platform (listservs, BB forums, Livejournal, Myspace, Facebook etc.)
Sometimes I also do see things in the “suggested ads” that I want to buy.
Sometimes they post some real howlers like constantly showing me “Christian match” ads in the past when my profile said “Married” and more recently, showing me an ad for a book called “Handbook for the Recently Deceased” right after I posted about a death in the family.
If one is nervous about being “spied” on by businesses or the government, then go live off the grid. I don’t put anything on Facebook that I care if anyone else knows, sees, or puts under the glass on the desk of the FBI director. As the guy from Sun Microsystems said, “There is no privacy on the Internet. Get over it.”
My husband chose not to have one because he had government clearances that would have required him to report things like contact with foreign nationals (we have a number of friends in Europe, Canada etc.) Which was fine, it’s a personal choice, but he did make very heavy use of a locked BB forum that most of our friends were on.