@Greyling, The best way I've found to get sim friends to know each other is to regularly host events where you invite those friends, so that they spend time socializing with each other. The feature that does it for neighborhood stories doesn't seem to add the relationship bits at all, at least on the ones I have followed up to look at. Either that or it's such a small amount that relationship culling axes it before I ever looked.
Clubs are also very useful for this, if you put them all in a club and associate the club hangout with a lot type which you regularly visit with your sims then those friends in that club will regularly gather at those locations even without your intervention. The played sims don't need to be in the club for this, just all the unplayed ones who you want to stay friendly. Set socialization as one of the club activities, visit their hangout lot type, and those sims will become friends on their own.
Speaking of relationship culling, if you use MCCC you may wish to tone down the rate of both relationship culling and relationship decay if it is important to you that your Sim's friends be friends with each other as well. Sims that are not played just don't socialize enough to maintain many close relationships with other unplayed Sims with the game's default for those.