Now I've completed all the activities in the city except for the tags, but is the loan sharks an actual activity? if so, what is needed to complete that activity? or is it a stationary place where you can just get money when needed?

You just get money when needed, you cant complete it since it is not an actual activity

I was wondering about that, because when i filter the activities on the map, the loan shark activity is on there with the other like the chop shop. So i take it that it has no effect on getting 100% in the game?

well when you take 200,000 out you get the acheivement

if you dont give back the money the guys will kill you but its fun i do it all the time