Some will look extensively once the find someone that they like. This is because they're actually "buying" the agent's services. Sales pros long ago learned that people do not look for reasons to buy something, weigh those out on a scale, and then make an informed decision. Instead, they do it the other way around 90% of the time. They decide to buy something and then look for reasons to justify the decision enough to cross an invisible line where they pull the trigger.
In the case of finding an agent, the natural process is to open ones eyes to who is out there by not just looking online but at local signs in front of houses, postcards they may have set aside, and other places. They might trip across a video on YouTube that an agent did, then watch a few more and start thinking that they'd like to work with that person.
Once the decision has been made that they want to work with "that agent" they start the snooping to justify the decision. They'll google them up and look at their website, their linkedin profile, their facebook profile, their activity on Trulia/Zillow/Realtor.com, basically anything that confirms that they were right in their decision. They more they start to "tie" to an agent, the less likely they are to abandon their decision, even if they come across one or two negative reviews of that agent's service or reputation online.
After all, everyone has a bad day and not every transaction is going to be perfect and grumpy people tend to post and rant online more than happy customers - right?