Source: http://www.localseoguide.com/how-did-thumbtack-get-out-of-the-seo-penalty-box/?
Thumbtack.com received a manual penalty on the 10th of June which rendered the site all but invisible in Google’s SERPs. The site has experienced a seemingly complete recovery after this.
Recover From A Manual Penalty
Thumbtack’s 3 main sources of backlinks:
1. Quid-pro-quo links from Thumbtack advertisers
2. Links from advertisers trying to push their Thumbtack profiles to the top of the SERPs
3. Links from PR-related activities
How they got out of penalty in 6 days:
– Thumbtack had contacted its advertisers to try to get these links nofollowed and/or taken down.
– Disavow tool
This remains to be an interesting case study to date. People are hoping that Google would better give an explanation for the drop and the fast recovery.