2009 cultural debate: Was it more late 2000s or early 2010s?

To me it seemed it was only late 2009. Something like October to December that the 2010s vibe was creeping in.

Looking back at 2009, it serves as a bridge between the cultural 2000s and 2010s. It definitely wasn't core 2000s anymore but the early 2010s haven't begun until around late 2009. I will admit that I felt a change in culture around late 2008/early 2009 with Obama, recession, Facebook(Yes, I heard of Facebook when I was 10), touchscreens like the iPod touch, and electropop creeping in music; it wasn't the same 2000s as late 2003-2007. However, those changes were further amplified around late 2009/early 2010 when those things listed doubled in terms of popularity making 2008/09 feel closer to the 2000s due to many stuff from the 2000s lingering around. I would put 2009 with the same category with 2013 as a unique year that have a mixture of both eras.

The way I also see it, was that September 2007 to August/September 2009 were the late 2000s.

It started properly feeling this way to me (late 2000s) during spring or summer 2008 with identity starting to become clearer or stronger in a few ways. (Late 2006 to mid 2007 felt like mid 2000s that were starting to lean towards the late 2000s)

Late 2008 to mid 2009 felt closer to a new era, even though we didn't know what it was, and things starting feeling or seeming much more distinct from the vibe and style of the pure mid 2000s.

By Fall 2009, the actual real 2010s vibe was creeping much more but I think the era was slowly starting as well.

Even though this happened and all of it was felt at the time, it took until 2011 or so to make sense of it in a way.
Thats just my longer take.

Honestly, the most accurate breakdown of the late 2000s.

Late 2006-mid 2007 were moderately mid/core 2000s with modifications that felt somewhat different from the actual mid 2000s. The culture was still the peak of the McBling era where the luxurious maximalism was everywhere in brands, clothing and music. The fashion felt the same as late 2004-mid 2006 but with modifications such as brighter and flashier colors that gave fashion a break from the earthy tones of the mid 2000s. Music was also not very different from the late 2004-mid 2006 despite being more "digital". Crunk still reigned supreme, urban-pop crossovers were peaking than ever with acts like Justin Timberlake, Fergie, Gwen Stefani, and emo punk was at it's highest peak with acts like Fall Out Boy and My Chemical Romance. This period was also the Timbaland renaissance, he was absolutely massive. MySpace hit it's highest peak in terms of social media, Facebook was around but the media was more underground and considered inferior in this time. The biggest change was the technology with game consoles such as the Wii and PS3 being released and the iPhone came out in the summer of 2007.

Late 2007-mid 2008 were the proper late 2000s but still in the core 2000s range. I remember when Disney Channel was the rage of this time because of Hannah Montana, Camp Rock, and the debut of Phineas and Ferb. This would be the last period where crunk was on top of the charts like Low, Cyclone, and I.N.D.E.P.E.N.D.E.N.T. Music was in the 2000s but artists such as Kanye West and Britney Spears adopted the electro sound that are stylistically late 2000s and would continue to grow later on. Soulja Boy and Lil Wayne were the faces of rap in this period and would make a profound impact in the future of rap. MySpace was still king in social media but competition began when Facebook proved itself to be another legit social media platform. Scene started to replace emo and probably a backlash to the depressing and dark aesthetic of emo culture. This period would be the time when the Recession became an issue but it wouldn't be felt until the next period. 

Late 2008-mid 2009 had a big change that serves as a preview of the upcoming decade and felt distinct from the mid 2000s. Barack Obama was elected President of the United States and a huge milestone for the obvious reasons. The impact of the Recession felt like a bang that was heard from the entire world. 2008-2009 felt like a tug-a-war between Facebook and MySpace, it seems that the latter didn't have much ground to stand on since the former was rising in great proportions. The big changes were when pop music slowly transitioned to electropop with acts like Lady Gaga, Black Eyed Peas, and Britney Spears. Rap had a poppier sound compared to the previous year with acts like Kanye West, T.I,  Flo Rida, and Black Eyed Peas were making pop friendly hip hop. Even though smartphone ownership was very rare, the Blackberry phones peaked this year and people began to buy iPod touches which slowly began app culture. Scene started to replace emo and was probably a backlash to the depressing and dark aesthetic of emo culture. 

Late 2009-mid 2010 would be the official start to the cultural early 2010s. The party atmosphere was in full-effect to cope with the recession times. Electropop reigned supreme in the charts. Most of the R&B even had the electropop sound into the production. There were also newcomers that would define the 2010s such as Justin Bieber, Drake, and Bruno Mars. There were still people wearing 2000s fashion but the trendy ones were in the 2010s range. Neon flannels were the rage, girls started to wear longer and straighter hair, wearing boots and skinnier pants. Facebook defeated MySpace as the king of social media leaving Myspace lying down while slowly dying. Mobile gaming officially began with the rising sales of iPhone/iPod touch and other touchscreen alternatives.