Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What's happening: Shares of Twitter (NYSE:TWTR) were up 16.4% as of 11:40 a.m. Friday after the social networking site reported better-than-expected fourth-quarter results on Thursday.

Quarterly revenue climbed 97% year over year to $479 million, which translated to adjusted net income of $79 million, or $0.12 per share. Adjusted earnings before interest, taxes, depreciation and amortization rose 216% to $141 million. Analysts, on average, were only looking for earnings of $0.06 per share on revenue of $453.1 million. 

Twitter also expects current-quarter revenue in the range of $440 million to $450 million, with adjusted EBITDA of $89 million to $94 million. By comparison, Wall Street was modeling earnings of $0.04 per share based on revenue at the high end of that range.

Finally, Twitter sees full-year 2015 revenue of $2.3 billion to $2.35 billion, and adjusted EBITDA of $550 million to $575 million. Analysts were modeling 2015 earnings of $0.34 per share on sales at the low end of Twitter's expected range.

Why it's happening: "We closed out the year with our business advancing at a great pace. Revenue growth accelerated again for the full year, and we had record quarterly profits on an adjusted EBITDA basis," said Twitter CEO Dick Costolo. "In addition, the trend thus far in Q1 leads us to believe that the absolute number of net users added in Q1 will be similar to what we saw during the first three quarters of 2014.

To be sure, even after accounting for a loss of roughly 4 million net monthly active users in Q4 due to "changes in third party integrations," Twitter's average MAUs rose 20% year over year to 288 million. Average mobile MAUs also notably comprised around 80% of the total. Better yet, timeline views outpaced growth in MAUs, increasing 23% year over year to 182 billion, while advertising revenue per thousand timeline views jumped 60% over the same year-ago period to $2.37.

As it stands given Twitter's impressive fourth quarter, solid guidance, and momentum headed into the new year, it's unsurprising investors are so aggressively bidding up the stock today.