Hey there - long time no see 
Thanks to MmeLindor for giving me a nudge about this thread.
IMHO Social media is changing all the time. Free platforms need to make money somewhere - I mean, why would they provide a popular service to everyone and not want to profit from it?
Facebook is the main culprit at the moment and I'm just getting my head around "edge rank". This is basically what Mme's link is about - your post will only be seen by the people who Facebook believe it is relevant to. They have their own algorithm for that and you can only extend the "reach" if you pay. Bad news all round basically, especially if you don't have the time or the inclination to invest in "professional" social media promotion. If you don't have a facebook page for your blog now then don't bother starting one.
Again with Facebook, using third party apps to promote your blog posts (eg. networked blogs) is going to take away some of your edge rank (visibility) too. The best thing to do is to be YOU on Facebook (your profile or your page if you have one) and post links manually. There is great discussion at the moment about whether you should try and promote your posts with an image - after all... isn't that what Pinterest is about? But then someone pointed out the other day that their Facebook feed was all pictures and pictures-with-words-on and crap - she just wanted to interact with people with actual typed words. And there - right there - is your audience feedback.
There has been a recommended app to use called IFTTT ifttt.com/ - but I haven't used it as I'm trialing the manual approach at the moment which seems to be working for me. However, you have to find out what works for you - especially when time is a massive issue for us all.
Twitter, for me, is like MSN on speed. They only people that will see your updates are those who are there at the same time as you OR those who actually scroll back when they come on line OR those who have you on a twitter list (worth looking into btw). In my mind, if you want to promote your blog then do so regularly. It's recommended to do it three times a day but I say more. You can schedule tweets with Tweetdeck or Hootsuite so you can promo even when you're not there. It takes 2 mins to schedule five tweets.
But G+ is the BIG thing for me at the moment. I love it over there. I'm finding a brand new audience and the stream is a lot "quieter". Depending on who you follow and what circles you group people into determines what you see in your live stream as you can "mute" certain circles, you can tag full circles so that people are alerted to your posts but the MOST important thing is that SEO is built into your Google+ posts automatically the more you post on there, the more chance you have of having your posts and updates seen on Google searches.
A +1 is like a recommendation. Your influence pushes it up the google search rankings a little bit. A share gives it even more of a boost. If you apply for Google authorship plus.google.com/authorship your recommendations have even more weight. You can create a Google Plus Page for your blog/brand/business and currently this has more SEO built into it BUT it's not necessary. I personally post and share and recommend in my own name because that's what I want to be recognised - however, for anonymity reasons or separation issues or whatever - some people want to concentrate on their pages rather than their personal profiles. Setting up a page is slightly more time consuming to start off with (and you need a G+ personal profile to start off with) but I have seen a few blogs and brands grow enormously.
At the end of the day - this is all only relative if you have the time to invest into it. Once everything is set up, it only takes a few minutes a day but you have to be a communicator and conversationalist (responsive) rather than a broadcaster (updates only). But if you blog for the love of writing/blogging and don't want to get involved in all the social media promotional side of things then that's fine too.
Hope that helps.