Threads , [WebSocket & nodejs] vs [asp.net & signalR] clarification?

In a situation where a ---

• Client connects to an asp.net (which uses the signalR FW) and upgrades the connection to a webSocket connection(tcp)

or

• Client connects to a Node.js server (and also upgrades the connection)

Question :

If the server now wants to initiate message sending , does it dedicate a new thread to do it ? because if it does , I can't see how signalR||Node.js is scalable because sending a message to a 100K users , can crash the system.

or am I missing something :-) ?