A lot of the kids shows use "pearpad" instead of "ipad" (don't ask why I know that) just have to come up with clever spins on the names. Could be half the fun of the game
There is a perception that they change the names because they have to (as in Apple would get upset with them). That's not true. The reason they change the name is because they would be giving free advertising, which would upset the people paying for advertising.
Imagine you were Samsung and were paying for commercials on a show that kept talking about the iPhone, you'd be pretty upset that they were giving free advertisement to a competitor. If you were also talking about products for free it would make it very difficult to get paid for product placement.
If you want to write a game where one character says to the other "I was on Reddit the other day and..." that's fine. Reddit doesn't have the right to prevent people from talking about their site. You could even sell your game and criticize Reddit if you wanted.
What you can't do is take someone else's ideas and use them as your own (fair use exception exists though). For example if in your game you created an internet forum with fake web postings and called it Reddit, you are potentially now infringing (you created the impression that this was Reddit).
It should be noted that a company could always threaten suit, even if you are in the legal right. Many people/groups will avoid any content that even has the possibility of infringement just because it isn't the risk/cost of legal action.
All of the above is based solely on American law and my experiences with commercial fiction and what may and may not be talked about.