I think that social media is increasing the size of the language. New words are constantly being added. Either out of necessity due to the invention of a new piece of hardware or the emergence of a new tech company, app, or website. "Instagram" or "selfie", for example.
With that said, though, I also think that social media has shrunk the language at the same time. Not in the sense that there are fewer words out there. That I don't think. Fewer used words is where I'm going with this. That's because I believe that social media, forums, texting, etc. has caused and encouraged users to say the same amount of stuff with fewer characters. That has led to an over abundance of abbreviations and acronyms entering regular everyday language. I grew up in a military household, so I know lots of acronyms, but nothing compares to the plethora of acronyms that exist now. Add to that the entire emoticon emergence, and I think it is possible to send an entire sentence without actually typing out a word.
I also believe that social media is having an adverse affect on grammar and spelling. I can give an easy grammar example. Twitter and text messages limit the number of characters per message. That means things like commas, periods, apostrophes, etc. are characters that take up word space. I see people choosing not to use those things more and more often. That's okay . . . to a certain extent. But I have had students turn in papers to me that do not contain any punctuation or capitalization. That makes for some very difficult reading.
I see social media creating new meanings from old words. Take "troll" for instance. No longer is it a mythical creature that lives beneath bridges. It is now a person that intentionally makes offensive and provocative comments online.
I also see some Orwellian tendencies being written in social media. "1984's" whole doublespeak is something that I see in certain areas. In the book the example was "good" and "ungood." The point being that "bad" was no longer a part of the language. It was more "streamlined" and "positive." I see Facebook doing this with "friending" someone and "unfriending" someone.
My major in college was English. I don't see any of the social media changes as good or bad (mostly). But I do see the language changing in certain areas, which is what language does over time. It always has, and it always will.
I definitely agree that social media has an enormous impact on language. Even emoticons and emojis (^_^) are replacing actual words as a form of non-verbal communication. Acronyms such as LOL (laughing out loud), OMG (oh my gosh), and TTYL (talk to you later) are commonly used in all social media platforms. Even inventions of new words such as selfie are added to urban dictionary by the minute. The word 'selfie' was even Oxford Dictionary's Word of the Year once. As far as international languages, social media provides a platform to interact with people all over the globe and absorb not only their language, but their customs as well.