Get a PS3. It's pretty cheap and has a huge list of great titles (most of which you can buy pretty cheaply now). The PS4 will likely end up with just as many great titles, but it takes a while to accumulate a library of that size.
To answer you last question, most titles like FIFA are locked to their respective gen. There are some exceptions, but usually only in games where it's a necessity (such as FF14). The PS3 community is still very active though, and match making algorithms are (usually) a lot more efficient than they were in the past. There should be no issues finding people to play with.
On a side note, if you do get a PS3 i'd definitely recommend the Ratchet & Clank Future series. Has been one of my favorite games series since i was a kid ^^ Ain't nothing as satisfying as forcing your enemies to dance with the Groovitron 
