Watch out for "Google Hell" by association
Andy Greenberg recently published "Condemned to Google Hell" in Forbes Magazine. In the article, he discusses certain SEO tactics used to increase Google ranking. That is, until Google discovers the tactic and blacklists them. It chronicled several companies that were getting page one exposure and making serious money because of it. That is, until Google found out and steered their gravy train right off a cliff.
But that couldn't happen to you, right? You don't use link farms or create bogus content. Well, it might just happen by association.
That's what just happened to me. I advertise on about 15 different web sites to generate leads. My #1 lead source was a company called VacaPro's* (name changed to protect those already doomed to Google hell). VacaPro's* were consistently near the top of Google organically for my search terms (Seattle vacation rentals, Seattle vacation homes, etc.) When a customer clicked on VacaPro's* (which they did very often) site they would see my properties and make a reservation. It was good times.
Then, one day in early April, it all came to a screeching halt. VacaPro's* was (and still is) completely invisible to Google. I thought it was an anomaly for a while. But today I called my sales rep to see what was up. In addition to letting me know he was leaving the company (he has car payments to make, you know), he said that they had indeed been sentenced to "Google Hell." He wasn't exactly sure why they were blacklisted, but said he believed it had something to do with their linking practices. The company is frantically trying to figure out how to fix it. Just to give you an idea of how important Google is for a business, check out my lead totals February through April.
February 641 leads March 764 leads April 173 leads
I'd call a decrease in 500 leads/month pretty dramatic. Fortunately for me, the company is one of several advertising sites I use, so it didn't have a large impact. However, for some, it was their main source of business. My sales rep remorsefully talked about many of their customers were going out of business/laying off employees due to loss of VacaPro's* Google exposure.
Here's the real kicker. As business owners, we really have no idea how a site gets top Google ranking or if/when it will come tumbling down. You could be banking on sales achieved by high Google organic ranking that could instantly run dry. I'm glad that I didn't put all of my eggs in one basket. It could've ruined my business. I'd recommend that for other business owners as well.
I hope this has been a helpful warning. I don't want anyone to get caught off guard.

