Alright, so I made a quick graph here -- there is quite a bit of history behind this and it will explain the crazy jump around 2008, haha.
2004: The year that I joined the community. I started playing around the middle of September; I had some rhythm game experience playing DDR on heavy mode quite a bit in the past, but DDR is quite limited in terms of what you see in speed, so that obviously did nothing for me. My stamina was terrible, because I played one-handed, using only index/middle. However, because I was used to playing DDR on a PS1 controller to test out my edit-mode stepcharts, my accuracy wasn't half bad. Unfortunately, it didn't help much.
2005: 2005 didn't see much of a difference. My speed had increased ever so slightly, and it was noticeable enough to where I was reducing miss-counts in the actual DDR game (on controller) by a few here or there, but nothing was really noticeable.
2006: 2006 was the year where I started to practice index. It was a bit more practical as a pad player, and some of the more complex patterns in pad-like keyboard charts were a bit more easily doable. As a result of getting somewhat attuned to the motions of using only one finger on each hand, I received a noticeable improvement. I was able to FC many of the boss songs on DDR that I was never able to do before. This is probably when I hit D2 level on FFR, because I was actually able to contend with really streamy songs like CIA Rave/Gradeus. 2006 was also when I learned how to use the StepMania editor, and I eventually submitted my first keyboard simfile to FFR, Coactive.
2007: In 2007, there was a somewhat extended period of time where I had no internet (prior to this, I had dial-up, so no internet was only -slightly- worse than dial-up). A friend of mine that I got playing FFR found out about StepMania, and actually introduced me to it. At that point, I started downloading some of the older Community Keyboard packs and really found myself improving drastically. Files like Air and AA were very easy to AA, and even faster songs like Ahh, Das Ist Musik were becoming doable. This ended up being where the major skill progression started.
2008: In early 2008, I was still without internet. I kept getting newer packs and started realizing that index wasn't working very well and I was struggling to keep up with dense areas that had large amounts of jumps into them. I hadn't really been involved with the community to consult them about spread, but I picked it up on my own after realizing that certain rhythm games have your hands spread out in the fashion that they area read in (IIDX is a fantastic example). Once I figured out spread, my skill increase was absolutely ridiculous. I left my debit13 account after making the cut in Team Chaos, and created a brand new account to post in the forums. Surprisingly, I wasn't met with BS calls after obtaining Oni within the first 24 hours of making the new account. As 2008 went on, my skill continued to improve dramatically -- I joined the 4th Official FFR Tournament and made it all the way to round 7 before being defeated by -Laggy- on Hardkore Atomic by a fraction of one good. Between the start of the 4th official tournament to the end of 2008, I nailed a shitload of FMOs and snagged Reality as my first FGO.
2009: 2009 continued the constant improvement -- I managed to get Tier 0 status, AAA all 11s and down at the time, hit average rank 1, and even pull off some stupid SM accomplishments like getting 1 miss on Disregard. FGOs were becoming a bit easier and FMOs were pretty much instant AAAs unless they posed some pretty extreme technicality (He's a Radical Rat, for example, was something that gave me issues for a while).
2010: Because of the huge FFR downtime, I stuck to SM for a while. I started playing (and inevitably creating) dump files. My stamina, surprisingly enough, didn't ever really feel like it improved. However, my speed took a noticeable increase. When FFR came back, low-end FGOs were no longer an issue and many times, were done with very little effort. FMOs were guaranteed AAAs no matter where they were, although I'd occasionally find myself restarting once or twice just to make sure I got the gist of things.
2011: Not too much happened in 2011 in terms of major skill progression - I just continued to play and saw minor improvements on my FFR game as a whole. My scores started getting better and I started running out of things to play.
2012/2013: The graph states 7.5 for FFR skill, but I like to attribute that to the end of 2012 and the start of 2013, where I started playing competitively again. The lowered values for 2013 are how I feel about myself right now, because I haven't been playing nearly as frequently as I used to, and I feel like I've taken a very minor skill regression. In any case, late 2012/early 2013 saw some pretty insane scores: 1-0-1-0 Rave7 (2-0-0-1 FC), 1-0-0-1 on do i smile?, 1-0-0-0/AAA on Jamais Vu, AAA on Extratone Pirates, and even a few random quads on SM (Sail Away being the most notable one). Songs that used to murder my hands, for whatever reason, didn't do so anymore. I was able to really push myself to some stupid limits. My SM accuracy took a hit, but it's mostly due to the fact that I'm pretty much 100% FFR exclusive. I'll even convert SM dump-files to FFR.
I'm done babbling, but I hope this was some good insight for you guys, haha.