Will expired Pepcid (famotidine) still be effective?

Yes. Most medicines may still work years after the expiration date, but the effect may be weak or unpredictable. There are few medications that never should be taken after the expiration date ( example : : tetracyclines. Avoid liquid antibiotics, insulin, & nitroglycerine).If the expiration date passed a few years ago and it's important that your drug is absolutely 100% effective get a new bottle.
Who knows? The expiration date is the date which the manufacturer guarantees it to be effective. It may still be effective beyond that date.
No. Expired products will increase acidity and oxidative stress you will pay by hurting liver and body aches - not worth it.