On the 10th of September 2015, Midgard announced that he'll be gradually winding down the forum by the end of the year, after a long nine years of existence. One of the main cited reasons for this decision was the relatively low user traffic on the forum, and the often lacking reader feedback to the timelines and stories of the resident writers.
The Official CF.net Refugee Thread - Thread from 2010, when CF.net was down for repairs for a while.
It's official: CF.net closing its doors - Discussion on Midgard's CF.net closure decision made in September 2015.
Anyone know if CF.net's successor site is up yet? - Discussion on the successor site for C-F.net, from February 2016.
Counter-factual.net received something of a spiritual successor discussion forum (as mentioned in one of the above included discussion threads). The Writer's Ark discussion forum was launched in early 2016, and is aimed primarily to foster writing and posting of stories, and discussions on conceptualising, writing and publishing various fiction (including speculative fiction genres, of course). Though not specialised primarily in alternate history fiction, AH is one of the many genres you can discuss over there, in the capacity of an AH writer.