Psy's "Gangnam Style" video has been viewed 2.6 billion times (it broke the YouTube counter). Mind that this is YouTube rather than Facebook, though much of the video's sharing was
through Facebook. So the capacity to reach some billions of people is
possible and
has been attained.
https://en.m.wikipedia.org/wiki/Gangnam_Style
I'm noting this as an example of a widely-shared piece of content that I'm aware of. I'm also having trouble turning up widely-shared content Facebook-specific stats.
"An Anatomy of Large Facebook Share Cascades" mentions a case of 600,000 re-shares. That's a substantial fraction of total Facebook users.
https://www.facebook.com/notes/facebook-data-science/the-ana...
A listing of ten-most-popular Facebook posts of 2014 gives values into the 3 million range.
http://www.hlntv.com/article/2014/12/22/facebook-most-shared...
What I've looked at is the distribution of posts on specific topics of intellectual interest, as compared with nonintellectual discussions. Total community size matters, though it's not a total determinant. Smaller communities with good discussion characteristics (say, Reddit) come close to matching Facebook on actual discussion, and very small communities such as Metafilter actually have very high ratios of relevant discussion, though these may omit specific domain or topic experts.
https://www.reddit.com/r/dredmorbius/comments/3hp41w/trackin...
On a similar basis, if you want to find any one particular subset of people, you're in general going to find that in a larger rather than smaller network. Of course, if you know which specific group you're trying to target, then there may be other networks of specific interest. In the case you make, of seeking out family, friends, or shared interests, in general, Facebook is that network, and the attempts of other networks to challenge Facebook on the basis of social graph has typically failed. Challengers who have succeeded have generally done so on other bases -- intrest graph (Reddit), specific subset networks (e.g., Slack), or 1:1 communications (Snapchat).
Of material I've posted for which I have available stats, I'm aware of reach on the order of thousands to tens of thousands of unique readers, and no, not on Facebook (I don't make use of the platform). That's pretty good, though I find that where there are very specific people I'm interested in corresponding with, reaching out to them specifically (usually through email) is the most effective means.