It's a very thorny area, and not one that I'd want to get involved in either. The FAA also has the concept of a "Common Purpose" to the flight. By my non-lawyer reading of this, it would be illegal for a private pilot to even have a casual conversation with someone, discover that they're going to the same city, and split the expenses. They have to be say, all going on the same vacation together. I believe that the intention of this is to avoid people flying with private pilots that they don't personally know and have the opportunity to evaluate their general judgment.
Yes, it's a good idea to be wary. As with most regulations, you'll get by with it as long as nothing goes wrong. The problem is that there are a lot of things that can go wrong.
A few years ago, I read about a case involving a friendly fellow (private pilot) who received a call from his neighbor in the night about a sick relative. The two were friends, so the pilot offered to take the neighbor by plane to save time. The neighbor asked if he could pay to help offset the expenses, and the pilot said that the only thing legal would be to share fuel expenses (split 50/50). At this point everything seemed fine. However, the neighbor wasn't accustomed to the high cost of fuel in aviation and decided to check if the cost was reasonably calculated. Since he didn't want to appear accusatory, he called someone he thought would know, and this happened to be the FAA.
The FAA saw this as him asking about the cost of the flight, involved themselves, and ruled that this flight did not meet the "Common Purpose" criteria, and it ultimately ended in an enforcement action.
Here is a link to an FAA letter on the subject of ride boards. Unfortunately, it doesn't bring a lot of concrete clarity, but it definitely makes it clear that there it would be easy to cross the line with these:
