Don't take your friend to an activity that he doesn't like or your 'like stat' will reduce.
You can perform upto four activities on each outing in order to give your 'like stat' the highest possible boost.
The other three girlfriends are met online from the dating websites, as
Niko Bellic can connect to the site using the in-game Internet to choose to date women.
The first type of relationship is any that Niko has with various females he encounters.
These relationships take on a more light-hearted and pseudo-romantic tone and are completely optional for the most part.
The Wikipedia links in brackets refer to the real life websites it is believed the game's websites parody.
The following links should not lead to the actual websites, but Wikia articles on that topic. The game's traditional news websites (discounting LCPD's website and the secret blog) change as the story progresses, many of their news articles documenting missions Niko does in the game and providing backstory or aftermath to those missions.