i'm a ucsd freshman and i've seen a lot of questions about the social life at ucsd. i party pretty much every friday and sat, and can answer anyone's questions about social life, partying at sdsu, parties, frats, RSO's, etc. to the best of my knowledge.
This is what I have observed so far:
RSO's: Definately not as strict as sdsu, since i've never seen one there, but the rso's at ucsd are pretty laxed actually. i've been caught drinking 3 times and all let off, and my friend was caught smoking and made up a dumb excuse of burning weed incense and got off. the rso's were even quoted for saying as long you are not loud, they have no reason to investigate you for drinking, and my friends and i drink a lot in our room and have had only a few encounters with them when it was our fault (we left our door open).
sdsu: my twin bro goes to sdsu and i've partied there a few times. it's definately possible to party there and i've done i think 4 times, each time i've taken the bus and crashed at his place and bussed back the next day since i don't have a car. the parties are a lot harder to get into and you basically have to sneak into the good parties there, or go to a lesser known frat with a (not as good) party, which have way more guys than girls. the sdsu parties seem to serve mostly hard alcohol, while uc parties have a few kegs of beer. also, the sdsu parties usually have a dj with a dance floor inside, and people socializing on the outside. ucsd parties usually sometimes have dancing on the inside, but lots of beerpong outside and socializing both inside and outside the house. sdsu parties are almost always in frat row, while ucsd parties are house parties. sdsu parties also have strict doorman since everyone at sdsu wants to get into a party, while ucsd have usually no doormen and i've been able to walk into parties by the top frats with no problem.
parties: atleast 1 party every fri/sat night. i joined the sd board club and the surf club which each have a good amount of parties (especially the sd board club). also, open house parties come up on facebook and i usually snag a ride to one of those. the board club, plus surf club, plus open parties on facebook are enough to fill the weekends and so far there hasn't been a weekend-night without a party.
frats: i'm not in a frat, but my suitemate is and i plan on rushing one next quarter. the frats are much smaller compared to sdsu, and ucsd frats are about 60 men total.
cars: a car, or friend with car, is recommended for off-campus dining, concerts, house parties, etc. i dont have one, neither do my friends, and its been fine but taking the bus everywhere is a little annoying and frusterating at times.