I think all of us lie somewhere on a spectrum of how much we need the company of others and how much we prefer time to ourselves. Personally, I have realised I am happy doing things myself that most people are not from about the age of 6! I find protracted periods of company (like being on holiday with other people or staffing a school residential) extremely enjoyable but.........the word I would use is 'intense.' So on the spectrum I am definitely a good way down the 'prefer time to myself' end although of course no man is an island and too much alone time is bad!
Living on my own, I find the loneliest times can be weekends if I haven't arranged to meet anyone or even evenings after work. I definitely find Winter and dark cold evenings lonelier. Holidays from work (which for singletons can be our main source of interacting with others) can also be lonely. I tend to holiday alone (abroad!) and find the daytime fine but do find eating alone in a restaurant in the evening quite uncomfortable - I either find a restaurant that is quite empty (so they will be glad of any custom and you are not 'occupying' a table that could have had 2 people on it, one where there is at least another lone diner - where I will deliberately ask to sit close by so us singletons look more 'common and even sometimes in the hope of striking up a conversation and even eating with them - which can happen, or keep returning to a restaurant where it is evident they 'don't mind' singletons! Or if feeling more antisocial go somewhere with a takeaway like a pizza place! I do find long walks are therapeutic as they tend to put your whole life in perspective and allow you to mentally leave the 'here and now' for a few hours.
I agree about the FB comments - sometimes reading FB threads it appears everyone else is having 'the time of their lives' sometimes but as has been said, this is a biased self-representation of their lives and people have actually told me my FB posts make me look the 'adventurous one!' Remember at any one time most of your FB friends will not actually be doing anything of great interest and therefore not be posting about it!