This isn't social media's fault at all, snapchat has just revealed something that's been happening before the existence of social media.
Pre social media, the only way someone who wasn't at a party knew you were there is because of all the post party conversations and depending on the party, being there would increase your social standing among your peer group, e.g. "yeah I saw Matt in the cut getting danced on", this is no different than the getting likes on an IG video, only difference now is that IG is proof that "Matt got danced on by some chick last night".
People in here talking about fun are doing so dishonestly in my opinion, they may even be oblivious to it. "Fun" is a side affect of parties, when you cut through all the bullshyt, the purpose of these parties is to increase ones social ranking or use your present social ranking to receive benefits which may include access to women, sex, and attention, which is another reason why many of these parties are very exclusive, if your social ranking isn't at a certain level, you're literally not wanted at the party, and if you do manage to get in, you're practically shunned by the people with higher social rankings.
Parties are politics under the guise of having fun. Social media is simply a tool that people can use to increase and even artificially inflate their social ranking with the goal of receiving benefits based on that ranking.
The only thing "no-cellphone" parties would do is increase the number of people making pre-party videos. People would literally be snapping seconds before they had to give up their phones and once they did hand them over, they'd spend a lot of the time regretting it because they can't see how many likes, views or DM'S they may have gotten during that period.