I'm not limiting this list to the major players - I'll also include other websites that may either display some form of profile of mine, a little brag, or somewhere you might be interested in participating/visiting/dabbling yourself.
Occasionally I post something more than 160 characters in something labelled Thursday Thought, a regular jaunt for tangents. Sometimes there's some Friday Fun, and occasionally some science & other stuff.
-- nee triangle-circle-square.com
This profile seems to grow like a plant... cultivate a seed and with minimal effort the profile slowly grows into a CV that's informative, interactive, professional & contemporary. Note, I generally only add people to my network I've personally dealt with, or had frequent communications online.
It's been a long time weaning myself into this domain, even though I'm an avid user of Google products. I think when I was notified to consolidate my blogger profile with Google+ it was the push I needed to reinvigorate my activities after a little sojourn.
I find myself using this more frequently, in addition to using it to consume information. I'm using it in two major forms.
1) for work - I do follow many Oracle related tweeps, and I find it a handy distinction between my personal usage of Facebook
2) I'm someone you might term a skeptic or humanist - there are a number of people I follow in this community, and I also use it to post/vent various thoughts related to pseudo-science and other baloney. If this offends, just don't listen!
I believe I signed up for this before I heard of it so I could play poker online with an old buddy of mine - he said just sign up so we can play... thanks man.
Now I'm generally a watcher that passes judgement on others, but occasionally I'll find something worthy to post or think of something witty to add. It is now treated as another tool to keep up with randoms.
I don't typically add my work colleagues, you kinda have to be special for that ;-) - this is generally the domain of personal friends.
Oracle Technology Network (OTN)
Obviously I use this account every time I need to download a wonderful piece of Oracle software, but when I have the chance I occasionally participate in the forums when I need to stimulate my brain. In 2012 I clocked over into the "Expert" part of the spectrum, so I guess I'll be sticking with this forum - but stack-overflow is a handy supplement. Along with documentation sites such as Oracle, jQueryMobile & jQuery, hopefully soon PhoneGap ;-)
For a number of years while I was new to the industry, I spent a fair bit of time at AskTom. I think this gave me a good leap ahead of the local pack. While I don't find myself there too often any more - I have found new to sustain the brain - I fine the PL/SQL challenge valuable in keep my PL/SQL skills fresh, and occasionally learn a thing or two about functionality, performance, good practice - while enjoying a quiz. Steven has also expanded this to a weekly SQL & APEX quiz (although latter not always regularly available - I'm sure we'd all like to see membership grow ). It takes 5-10 minutes of your day, why not give it a crack? You don't need to be exceptional to win a prize - I received a prize in late 2011 just for participating (note, correlation does not mean causation!). I received a 10th place certificate in 2nd quarter 2011
While not formally social media, I do consider these aggregators as contributing to my presence on the web.
These websites consolidate a large number of Oracle bloggers, so if you don't want to set up your own RSS feeds, just visit these occasionally to catch up/keep up with the technological world around you.
orana.info - Oracle News Aggregator, arranged by Eddie Awad
apexblogs.info - APEX Blogs - thanks to APEX Evangelists (update - now managed by ODTUG)