> iPhone

I doubt it. How many high school seniors would have owned one soon after its release? Too much of an early-adopter luxury item.

I'd be curious to know when smartphones actually started became ubiquitous among older teenagers.

I graduated HS in 2007. I think I had 2 friends with iPhones, in an affluent town in Connecticut. My first smartphone was an iPhone 4 in 2010. It was still probably another year before smartphones were ubiquitous amongst my friend group.

The 3GS came out in 2009 and that's more or less the model that made it clear the iPhone mold of smartphone was the future. (I got a Treo in 2006 but that was still very much in the carrier walled garden on phones.) However, as others have said, feature phone texting was pretty widespread in the second half of the 2000s, IM was widespread, and social media was really starting to take off.

You can quibble about exact dates but it's pretty reasonable to take somewhere around 2008 as the year for online communications hitting some sort of collective inflection point.

I would agree that online communications hit AN inflection point around that time, and I don't think I said anything to refute that. I do think there was a more important one soon thereafter, because IM isn't what made me feel like my social life had moved primarily into the virtual space. It was several years after this, more like when Instagram took off. It was not until then that it became common for someone, or a couple, to be looking at their phone throughout dinner (I worked in food service throughout college), or for an entire group of people sitting around in the same room all interacting through their phones. Facebook, and other social platforms around 2008 set the stage for this but I think it's not until the smartphone got great cameras, fast internet, etc. that it became more convenient for us to interact through them. I think it's an important distinction. I was not quibbling, just providing an anecdote. I used AIM/MSN from a feature phone in 2007 and used SMS heavily prior to that.

Oh I don't disagree with any of that. For doubtless a variety of reasons (if only that habits don't change overnight), it took a few years after at least many of the pieces were arguably in place, before widespread behavioral change took place.

Dumbphone texting was ubiquitious, though.

Remember the teens with leet T9 keyboarding skills?

I graduated HS in 2014. By the time the 4s came out they were pretty ubiquitous. Carrier subsidy existed to make one or two year old iPhones pretty cheap (I vaguely remember iPhone 4 being 'free' from verizon for a while). I didn't get one until after many of my peers (iPhone 5 era), but I had an iPod touch and a phone with a full keyboard which was a common combo among the people I knew who were later adopters of smartphones.