We get out a lot. Even if it's just doing the shopping or taking myself out for a coffee. We have mothers group weekly plus group fitness training with mums and bubs twice a week. Then I have school runs, house work, the occasional catchup with a friend or relative as well as I pop in and have lunch or coffee with DF whenever I can and I haves few hobbies I come back to when I get a chance. So I think if I didn't deliberately fill my weeks I would get much more lonely.
I think somedays I feel it more but I have noticed it's worse when I feel a bit taken for granted or if DF and I have had a bad patch in our relationship. I'm not as happy in general so it highlights how isolating being a SAHM can be.