Don Quixote is the work that many identify as the first modern novel, a work distinct from epic romances or other high-brow treatments of fictions that were considered worthy of poetry rather than prose. Of course, that was not an English work, but a Spanish one. The novel genre only arrived in England nearly 2 centuries later, and often the stories were told in the form of letters.
Epistolary novels are set on the premise of the narrator, telling the story to a correspondent. Samuel Richardson opted for the epistolary form for both Pamela and Clarissa in the 1740s. About a half a century later, Jane Austen used that form in Lady Susan. Some decade or two afterwards, Mary Shelley framed the story of the archetypal mad scientist, Frankenstein, in her novel of the same name by having the story related by the sailor who picks him up on his sea voyage in letters to his sister. However, as the novel gener took off during the 1800s, most dropped the epistolary device, even if they were written in first-person.
It hasn't disappeared altogether, though, modern treatments tend to mix the letters with narrative, often from different points of view, as in The Guernsey Literary and Potato Peel Pie Society. Some contemporary novels substitute emails for letters and sometimes also throw in texts or social media style updates in telling their stories. I wouldn't be surprised if there is a novel that is relayed solely through tweets, possibly grouped under hashtags rather than traditional chapter titles.
But I wonder if anyone has attempted to tell a story through an online community bulletin board. I know that some reveal an awful lot about their lives through their posts -- about having children, having financial difficulties, attempts at getting a job, attempts at getting a loan, divorce, and calls for outright handouts. That's all from one person's posts over the past 3 or 4 years. For writers of fiction, I thought that such an account could form the central line of a narrative from which several key characters branch off.