The downgrades depend on the airport these days. In Schipol (Amsterdam) and most other European and smaller airports, FB sIlver will use the sky priority queue or Zone 2. In some larger airports like Heathrow (London) and Charles de Gaulle (Paris) we are designated Zone 3. However they usually just merge zone 3 with the economy ones so we are stuck in the normal queue. Like all of FB, Silver can take a guest with them in the priority queue even if they are not FB. My information is from asking the staff every time i get to the gate. Some of the info is conflicting but the info i posted is consistent from all of them.
Your priority status depends on the airport, so to get the most out of silver, it is best to fly to and from smaller airports and Sky Team airlines where possible. e.g. Use KLM instead of Air France and Delta. Fly from Schipol airport rather than Paris. etc.