First: (DO NOT OPEN if this is a really sensitive and touchy issue for you)
Spent this Valentine's day alone too, but over here where I live it's more of a day to celebrate friendship and showing your friends that you appreciate them rather than lovey-dovey couples running around on pink clouds.
The things I list aren't really specific to Valentine's day tho, they should work for any day of the year.
I guess one of the best things to do is to just think positively and if that really doesn't help, a temporary escape somewhere else than this world really helps fighting off the feeling of loneliness at least for me (roleplaying, listening to music, writing something, playing video games, reading a book, watching a movie... something that absorbs you and your attetion)
However, note that I said TEMPORARY.. If that ends up being the only thing you ever do the people you already have might disappear too. If things get that bad I'd suggest either therapy or talking about how you feel with someone you trust.
Imaginary friends (Yeah, you can be older than 5 and still have them! The "prototype" of Cheza started out as an imaginary friend and I was 14-15 at the time.. Today "she's me" and I'm 19) and daydreaming can also help sometimes, but that's also something that's mostly a temporary solution..
Isolation isn't good and will only throw you into a depression, and if it's gone on for a long time it might also have made you truly alone.
These are the things I (used to) do when I feel at my worst... Some things I mentioned could maybe help others while some things could maybe make people feel worse, depending on how they look at and think about things...
So yeah, mostly really speaking for myself here ^^' Hopefully it gives some kind of inspiration or something...