To use or not to use Data transfer objects(DTO) in a Web Api Chat Application Backend Server

I have a chat application backend built using Web Api where I am exposing several database entities directly to clients. I was wondering whether there is any positive points to map the entities to DTOs or should I continue exposing the entities as I am currently. Just to clarify I am not asking a DTO vs non-DTO general question but just advantages of using it in this scenario since most of the fields in the entities would probably be used by the client.

Push notification from an ASP.Net server

I have a web service that performs some operations. When an event occurs I will like to notify the clients. The problem that I have is that I am able to connect from the client to the server but not the other way around because clients happen to be behind a NAT (router). Currently I am making a request every minute to check for notifications. It would be nice if I can have a technique where I could notify the clients faster without having to make so many unnecessary request.