ok, i know you didn't ask for a comparison to xbox live but, honestly, xbox live silver > psn, besides the fact with xbl silver you can't play multiplayer games. just wanted to get that out of the way.
so, the psn.
the psn friend's list
there is a central list in the dashboard but, it is pointless in resistance since resistance friend's list is seperate. in other words, not every developer has to use the psn friend's list and format, they can do whatever they want and unfortunately, insomniac has elected to do whatever they want. (note: dev's can use the psn friend's list and i believe as time goes on more dev's will do this to make it appear more like xbox live. who really wants to have a seperate friend's list for every game?)
the psn store
it's ugly, sorry, but it is. there are only 3 things on the screen at a time and the icons are overly huge. currently there isn't any background downloading. also content is added nearly as fast as xbox live's content is. sure, it will take sony a while to get it started but right now, xbox live is leagues ahead of psn in content and store features/usability. there are some downloadable games, like xbox live arcade and nintendo's virtual console but, they aren't anything special and most are overpriced at $10.
messaging
pretty simple, text messaging, voice messaging. there's chat but i haven't tested it. i don't have a usb or bluetooth headset so i can't vouch for actually use of the voice chat features but, they're there.
that's pretty much about it. there isn't much to it. if you are looking for an online experience, xbox live will be the best experience, probably throughout this console generation.
of course i recommend (if possible) you try out both, xbox live and psn.