or, if we had developed it, but it was somehow not as addictive as it is...
Are you implying that social activities prior to social media wasn't addictive? It's more ubiquitous yes, but rumour and gossip of previous society is no less addictive. What can I say, humans like drama.
Would this generation's teens be as self-absorbed/narcissistic?
Every generation seems to ask this about the next generations teens...why don't you malted loving teens get off my lawn? Perhaps the correct question here is would our generations perception of the next generation of teens be as self-absorbed/narcissistic without social media, or would we find another culprit to blame?
Would society be more moral?
Less Moral, more blissfully blind. I would suggest the development of a global 'eye' that is the collective watching of the globe by itself through social media's has created a much more moral world, despite the perception. Take Ray Rice and the NFL on spousal abuse...are we saying prior to Ray Rice it's never happened, or we've never seen it/caught it on tape for all to see prior and even if we did, we lacked the technology to allow millions of views in under a day? I'd say more instances of immoral acts brought to light is a sign of a reduction of those acts as we are seeing them much more readily than we ever had before, not because it's suddenly happening now. A CEO was recently canned for a video of him kicking a small dog surfaced. Is this a sign of less Morality or a sign of the ever growing eye?...the fact that we are seeing them, in huge part due to social media, doesn't mean it happens more often. Teens committed suicide from teen bullying in the past, but never before has it become a social phenomenon that so many are aware of.
Teens today are very aware of world events and global issues. Would this still be the case without social media?
Yes and no. Teens are more aware of peoples opinions of world events and issues, debatable if that translates to actual awareness. Peoples willingness to research and learn global issues is pretty independent of social media, although social media does increase the availability of topics for people to look further into.
Anything else major?
Easier to control media. Before social media, 90-95% of media (radio, tv, written) was controlled by a very narrow band of people. Social media has brought about a rise of independent media (by allowing anyone to be a news source had they wanted), which has greatly reduced the blinders that single ownership media was starting to enforce. Back to the statement of morality...evil shit was much more easy to conceal prior to social media.
If we define the stack exchange network as addictive social media, then this conversation would never had occured either.
Editing from Michael's comment:
I have to strongly agree that Social media has brought forth a stronger generation of programmers deeply linked in a manner many of them feel comfortable in where they would have previously faced barriers in communication. Programmers tend to be notoriously introverted and sharing of ideas and problems wasn't as frequent as it could have been...now there is a ready line of communication between them. Technological advances, particularly in programming on SE, is a very strong example of what social media's can accomplish.
To go one step further...I've been watching dancing with the Stars and saw a piece on one of the people there, Bethany. If you search dancing with the... into google, it actually auto completes to her name as the first hit. She was the ultimate in introvert...as a child at school, she would answer questions directed to her by whispering to her mother and having her speak for her. These tendencies continued, until she started her own you-tube channel...a media where she could express herself freely. 4 years later, her u-tube channel ranks among the highest subscribed channels and is now a recognized name in over a million households. The classic 'American dream' lost to introverts is now a very possible reality.