What does "neeeeee" (ねー) mean when 2 friends are talking? That is, one is talking, talking, talking and the other will occasionally reply "neeeeee" to him? Is this just the ordinary "so da ne" or does it have another connotation in this case?
This sounds like it is related to the sentence ending particle ね。This particle, as you probably well know means something like:
isn't it, right?!, however sometimes it will be translated into English as an emphasizer, or an explanation point.
そうですね。(You're right! OR That's correct!)
今日あついね。(It's hot today, isn't it?)
Don't confuse this with the よ particle. There is a distinct difference between the two, even if they occasionally crop up in similar circumstances. You can read more about the ね particle here.
So, when we extend the ね to be ねー or ねーー、 I would say that an English equivalent would be extending
riiiiight?! In other circumstances where
right won't work as a translation, just realize that the speaker is emphasizing something more.
あのね、私はね、デートよ！今晩。(Yeah, I have a date! It's tonight.)
あのねー、私はねー、デートよ！今晩。(Hey, get this, This girl has a date. It's tonight.)
I think this example is pretty feminine though.
Lastly, ねーー on its own is pretty much a replacement for そうですね, as the other answers have explained.
What does the long “neeeeee” mean when 2 friends are talking?
If the pronunciation is a long "ねー", it means "そうだねー". There are times when you agree with the speaker or sometimes between people who hear what the speaker says. In these times, people who agree with each other will say "ねー" with nodding slowly at the same time with watching each other's eyes to show the deep agreement.
This way of expressing consent is often done between young women or between a mother and her child.
ねー is sometimes used as "heeey" like if you're talking to your friend and say something along the lines of "heeey, what's that?" (ねー、なにこれ？) or another example of ねー from the top of my head is when in anime 2 friends are like "something + だよねー！" would be -isn't itttttt? in my opinion it sounds more enthusiastic than just "だよね。" or "isn't it.”