My site raised ranks for nearly all its keywords this algo update but one. This one keyword (lets call it 'juicy melons') started dropping very quickly. As soon as I noticed I headed to the warrior forum and a few hours later people started asking about a new algo update. This confirmed my suspicions. I had seen a single keyword plummet in the exact same way before when I was new to SEO and made too many links with the same anchor text and an algo update came out. The pattern was all to familiar for me. The only SEO I had done on 'juicy melons' in about 6 months was to create a wordpress web 2.0, let the 2.0 sit for 3 months and schedule some posts. I then created another wordpress web 2.0 and placed a sitewide link to the first web 2.0. After 3 months I placed a sitewide link on the first 2.0 using 'juicy melons'. I then increased ranking for 'juicy melons' until this new algo update.
Today 'juicy melons' had reached page 18 from page 3 just a few days ago... wow!!! I decided the 2.0 experiment was over and to remove the link. It has been about 8 hours since removing the link and 'juicy melons' is on page 10 already. My site seems to move around Google very fast as it is an authority site. I make a post and it is literally in the index almost instantly.
I know this is not conclusive evidence however it is somewhere for you fellow webmasters who need help to start your research / testing from. I think an algo update hit web 2.0 sites. It would make sense as web 2.0 link spam has long had it coming to them from Google. The good thing is though you SHOULD have your log in details to amend the issue and remove the links with ease so if anyone can confirm my theory then you should be safe.
Please don't go rushing to remove all your 2.0 links! I could be wrong as my testing alone is not enough to prove this theory. Also I know my 2.0 strategy wasn't the smartest idea but I was experimenting. Experimentation can lead to some amazing discoveries and is great for reverse engineering Google.