You ask a great question. Yes, it is possible to combine other services or create an existing service, and you might still succeed. But to be accurate, almost all of the companies that you mentioned did in fact solve a problem and/or have something unique about them that was unique enough to bring users.
For example: Skype was among the first systems at the time (being first...), Signal skyrocketed recently because they provide complete privacy which became a big issue after FB announced that they were officially getting all the info from users on Whatsapp (so privacy = unique), WeChat was targeted at the Chinese market at a time where the other big players weren't in China (so right time at the right place = unique).
Bottom line, you either need to solve an actual problem/pain, or be extremely lucky and be in the right time at the right place and have the right connections/community.
Good luck