I used to get a lot of solo miles in, worked odd hours and split shifts so all my nine to five pals were unavailable. Once you are confident in your skills and knowledge of the trails solos can be some of the best rides. I miss the rides where there was no one to worry about, no on to talk to, just me, my bike and the trail. I never had any major mishaps, making sure I was riding within my abilities, our trails are pretty technical in spots, and made enough noise to keep the bears away, usually. I always made sure that someone knew where I was and when I should be back, plus I carry a cell phone in case i decide to go longer (or taco a wheel). I did use a GPS for a while, when I first moved to town, which made route finding a fair bit easier.
These days rides are much more social, my schedule is pretty set, very few mid day rides, so I only get one solo a week when I am lucky. My g/f loves to ride so we ride together 3-4 times a weeks and other friends join us from time to time. I guess I am pretty lucky.