For a brief period this month an electronic form of currency called bitcoins were all the rage. Promoted as an alternative to paper or "fiat" currency, bitcoin caught a popular wave with the public and became the it investment of the moment. With high profile investors like the Winklevoss twins, it seemed like nothing could stop the momentum.

Then as quickly as it inflated, the bitcoin bubble burst. After rising from $20 in February to $266 on April 10th, bitcoin crashed on April 11th, dropping more than $100 before trading was halted. The crash took the price of a bitcoin down more than 75% from its highs to the lows. As usual, it's the latecomers who are getting hit with the biggest losses.

Bitcoin will make a nice cautionary tale someday, but so do stories of the housing bubble, the Internet bubble, and every collapse since Dutch Tulips in the 17th century. Some even say the decline of Apple (AAPL) shares and gold's recent price slide are indicative of modern-day bubbles.

If the masses could really learn anything from these disasters then bubbles would be a thing of the past, rather than regular occurrences. Most people just can't help themselves when tempted with the prospect of easy money.

That doesn't mean you have to be the next victim of a bubble mania. In this edition of Investing 101, Robert Luna of SureVest walks us through 5 signs of an investment bubble.

1) Valuations deviate significantly from a long-term historical average

Facebook (FB) is still the most important social network for U.S. teens, but watch out, Twitter is quickly gaining share according to Piper Jaffray’s semi-annual teen survey. Among U.S. teens, 33% rank Facebook number one, with Twitter coming in a close second place with 30%. Most notable is the change from the previous Fall 2012 survey showing Facebook at 42% and Twitter at 27%. Instagram rounded out the top three with 17%, compared to 12% in the Fall.

“It is ultimately a long-term problem for Facebook,” says Gene Munster, senior analyst at Piper Jaffray, before reminding us that Facebook owns Instagram, so the two can be viewed collectively. It was last April that Facebook announced its $1 billion acquisition of the photo-sharing app company.

From that perspective, Facebook & Instagram combined lost 4% share to Twitter from the Fall 2012 to Spring 2013 (see chart below).

“Ultimately, near term there is still a big opportunity for Facebook to continue to add new ways to monetize the existing base; they’re under-monetized today,” says Munster. “This isn’t something you’re going to see in the numbers in the next couple quarters, but I’m sure if you’re Facebook and you’re looking at these trends, you’re probably taking notice and trying to figure out how you can navigate this brand amongst teens.”