I've recently found some websites which provide a specific number of visits by paying. I'm curious if this is a (very) bad practice for search engines (like Google).
It's not a bad practice, you can buy all the visitors you want. Most companies that offer these services will run a program connecting to your website through proxies increasing your unique visitor count. Though the visits will be less than a minute with 100% bounce rate.
It does nothing for your rankings and ruins your Analytics data. It's a waste of money and you are not having real people visit your website so expect no sales and no clicks on any ads.
I have never seen any evidence that Google uses traffic to a website as a ranking signal. There is certainly a bunch of speculation about it in this thread on WebmasterWorld.
If they did, they would have to very careful about it. Google is the main traffic source for most websites. Using website traffic as a signal would have to exclude traffic that Google itself sends, otherwise it would be a self-reinforcing feedback loop where Google sends more traffic and sees the website has more traffic, so it sends even more.
There is also the issue of where Google would get the data about which websites have traffic. Google has several possible sources for this data:
- Google Toolbar
- Google Chrome Browser
- Google Analytics
- Google's malware blacklist
- Google's DNS servers
However, they might be missing out on certain demographics such as Internet Explorer users.
You could certainly make the case that Google should be ranking sites better because the are getting users. Having users seems like it could be a sign of quality. A couple years ago I remember reading about an online retailer with a huge ad budget that was trying to make the case that their large ad budget should be considered a sign of quality that Google considers in its rankings.