How to deal with an ex you haven't interacted with for a long time?

Recently, I saw my ex on Facebook and she added me. I am hesitating to respond to her as we had broken up before due to her cheating.

I don't understand why she's suddenly coming back into my social life but we haven't spoken in over 10 years. Not interested in getting back together but am thinking about letting bygones be bygones.

Any opinions?
How to deal with an ex you haven't interacted with for a long time?
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