I agree with what everyone else has said about finding an awesome activity you love.
My mom is a stay at home mom, except all of us kids are out of the house. She joined the welcome wagon club (even though she's not new.. they let anyone join to help out) where she met a ton of new friends. She also took up tennis, where she met other ladies with empty houses now that there kids are away.
Through the welcome wagon she also had alot of dinner parties, lunch dates and even joined a canasta group and plays cards for lunch once a week.
If anything, she's met alot of new friends all in similar situations which has really helped her alot.