Hi C., I'm an only child, and to be honest there are times that I wish I had a sibling growing up, but I also am thankful for being al only child, my parents financial situation woulnd't have allowed for me to do the things I did growing up if there had been another sibling to support.
I also look at my family, my grandparennts between the two of them have 7 children, with marriage and my cousins there are 35 of us! It's so much fun being around all that family, and because I married into a smaller family, by husband has 1 sibling, my son won't get to experience that big family that I had except for my extended family. Except that I have found out that my friends really have taken over that Aunt and Uncle thing, they spoil my son as if he was a part of their family! I have a few friends who are only chidlren and we treat each others children like our nieces and nephews sure it isn't "blood" but we love them all the same!
I think you have to look at yoru individual lifestyle, if you can afford anotehr baby and also your time, as our childrenget older they also demand more of our time with dance recitals, soccer, football games, etc... I see to many families not spending quality time as a whole family beacuse dad is running off to soccer practice while mom is running off to dance lessons.
A far as being lonley goes, be involved with things that your child will enjoy, because I was an only child and my mom was stay at home mom, she was able to go to my shcool outings, which to this day I have memories of, she was also involved with PTA, Girl Scouts, and as an only child I also made friends very easily because I had too, and as I grew older I would have sleep-overs, be involved in sports, band, dance, an other thinsg besides a sibling to occupy my time.
If your child is "bored" offer play dates instead of a baby if you are unsure!
HTH,