'platform' is often used when you are discussing computers, software or social media. It essentially means 'system' , 'method', 'framework'. Example 1: A:What computer platform does your employer use ? B: They use IBM mainframes with 3rd party software. The question was asking 'What computer system does your employer use' Example 2: "What social media platforms are popular ?" = "What social media technology/application/system is popular' (ie. snapchat/facebook,etc, etc) In your example: “It’s still a good platform to keep in touch with friends.” is referring to a social media platform/technology/software system/application like twitter, instagram,facebook,snapchat ... The phrasing is standard so not an idiom. Platform has several meanings and in your example it's just referring to social media.

It’s not an idiom. Rather, it is a figurative use of “platform.” Literally, a platform is something that you stand on, and which can help you reach a greater height. Figuratively, “platform” can be used to refer to other things that help a person reach his or her business or personal goals, such as “Facebook is a good platform for keeping in touch with friends.” Another example: “The lead singer left the band because he didn’t think it was a good platform for his goal of becoming a superstar.”

In literal terms, a platform is something people stand on, or things are built on. In this sense, it is probably used in the software sense. A platform is usually a program, or set of programs, that other programs can be based on, or run on. You could say that the operating system of a computer or phone is a platform, as apps and programs run on it. In your sentence, it is probably referring to a social media program, which is sometimes not so much a platform in the more technical sense, but it can be. e.g. Facebook has various apps and games that run on it. "I think that Android is a good platform. I like the fact that is more open for developers"