The effectiveness of social media depends on your influence there. Unless you are already popular in real life, you are very unlikely to get noticed on social media.
I have a Twitter account with a large number of followers (not book related) and so my Tweets can actually gain some traction, but most Twitter accounts have a small <1000 number of followers. If you Tweet about your latest book (or whatever else you are selling) that's only 1,000 people it goes out to, and most of those aren't likely to read it either. If you Tweet the same thing several times in a short period of time then that will be considered spam and you'll lose some of that small number of followers you already have. If you can produce a funny, clever and interesting Tweet that then goes viral, then you can attract more attention. That happens very rarely, and even more rarely does it happen more than once. I've had one Tweet that went very viral and I cannot actually tell you the reason why. It was the second time I had used it and it wasn't even retweeted once the first time.
Getting yourself a reputation, known for producing funny, clever and interesting Tweets will take you a very, very long time. The timing and day of the week you send those Tweets is important too, but you can be doing
all the right things and still
never get noticed. Facebook is slightly different in that people tend to scroll through, and skim read everything, but essentially it has the same problem: that in order to build up a larger following takes you many years and much hard work. A little easier if it is tied to a popular Blog. I think a Blog and Twitter seem to work well together, but that is a lot of Blogging! While I do see some popular authors who work that way (i.e. John Scalzi) ask yourself, do you really want to write books, or do you want to have a popular blog and be popular on social media?
Social media is a communications channel, not a marketing channel - unless you have a time-limited deal to communicate.
Brian is correct and this is how I see most authors using it, in the same way as the authors who have forums on Chronicles; not to advertise, but to communicate, to answer fan questions, to win friends and appear as a friendly human face (or not as the case may be.)