I think you need to apply convenient to something, like convenient for keeping in touch or convenient way to keep in touch. As written, there is an implied something that is convenient, so you could say that in speech but probably should not in writing.
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I think you need to apply convenient to something, like convenient for keeping in touch or convenient way to keep in touch. As written, there is an implied something that is convenient, so you could say that in speech but probably should not in writing.
Yes
it is fine
Smartphones are a convenient way to keep in touch with people.
The sentence is awkward.
Better ->
"Smartphones are a very convenient way to keep in touch with people"