As has already been mentioned, with the end of the geocities hosting service, a lot of fan-fiction sites will be lost. Now, there are hundreds, if not thousands, of hosting services out there, and a quick Google search should be able to point you in the right direction (I'd recommend asking friends who have websites if they have any recommendations, too, as it's always good to get an unbiased opinion). So, if you're the web-mistress/master of one of those soon to be defunct sites, it should be relatively easy to switch over and continue as usual, as long as you've got an up-to-date backup copy, or can create one while you still have access to geocities (the wayback machine is also very helpful if your site's already gone bye-bye).
Unfortunately, not all web-mistresses/masters are going to want to rebuild/move their sites, and this means that a lot of fan-fiction, including femslash, could be lost forever. Now that, as I'm sure you're aware, is a crime against nature, and should be prevented if at all possible. So I wanted to take a minute and speak to those writers/creators whose work might be in danger of disappearing and offer a few basic suggestions (nothing earth shatteringly new but sometimes seeing it written down can prompt action - at least for procrastinators like me).
Firstly, and most obviously, you have to decide if you want your work to remain available to the general public. There are many reasons why someone might decide to withdraw their work from the internet, and I'm not in any way trying to guilt people into allowing theirs to remain, if they'd rather it be deleted. So, if you're happy to see your work disappear along with geocities, you can probably skip the rest of this ramble.
If the only place your work is available on-line is on a site hosted by geocities, I'd advise contacting the site maintainer to ask if they plan to move the site to another web hosting service. If they are, you're covered, and again you can probably skip the rest of this ramble. If they have no plans to move, then it's time to consider your options and, most importantly, make sure you have up-to-date copies of all your work (it's a great opportunity to reread and correct your work, too, as time and distance are wonders for helping you spot the odd spelling mistake or botched bit of grammar).
Now, having decided that you want your stories etc. on the web, you're faced with two choices: create your own website or submit/post your work to an established archive (Personally, I don't consider lj or forums as archives, but that's just me).
Creating a website is as easy or as difficult as you want to make it. If all you want is somewhere clean and simple to house your work, you can probably learn all you need to know in under an hour, or simply use one of a plethora of programs to do it for you (I've never used any myself, but I've heard that they're quite easy to use). Again, using myself as an example, I'm not at all computer savvy, but I managed to find my way around html code, so I'm sure you could too.
(I'm sure we could rustle up some helpful links etc. to building websites if anyone is interested)
Posting/submitting to existing archives is even easier, but I would strongly advise you to take the time to carefully read the site's guidelines and take a wander through its existing works, to make sure it's the right place for you. Single fandom or pairing sites are often excellent, and offer a myriad or resources for writers as well as readers, but they do tend to have a shorter life-span than larger multi-fandom sites. Big, multi-fandom archives, although they have the potential to be around longer, can lack the sense of community and being a part of something that you get with smaller or fandom based sites. So, you weigh your options and take your choice, or just say to hell with choosing, and post/submit your work to both types of archive (I'd strongly recommend this approach). It's that simple.
If you know of someone whose work is about to disappear, but who might not be aware of the fact, as they're no longer involved in on-line fandom, then I'm sure they wouldn't mind a friendly heads-up. Badgering someone, however, will just piss people off, and have the opposite result, so please refrain. Also, if work seems abandoned, with contact details that are out of date, this doesn't mean that you can simply take the work and host it yourself (even if you do cite the correct author name). It's bad form and will soon result in you being blackballed by the femslash mafia... and, believe me, it's almost impossible to get horses' blood out of silk sheets.
All very obvious, I know, but as the clock is ticking, it's really something you should be thinking about and acting on now, rather than later.